feat(S1): Add typed exception hierarchy — ZammadException interface +…

src/Exceptions/AuthenticationException.php

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+<?php
+
+declare(strict_types=1);
+
+namespace ZammadAPIClient\Exceptions;
+
+/**
+ * Thrown when the Zammad API returns HTTP 401 Unauthorized.
+ *
+ * Common causes:
+ *  - The API token is invalid, expired, or has been revoked.
+ *  - The token was created for a different Zammad instance.
+ *  - The `X-On-Behalf-Of` user ID does not exist or the token owner lacks
+ *    impersonation rights.
+ *
+ * Resolution: verify the token in the Zammad admin panel under
+ * *Avatar → Profile → Token Access* and ensure the required permissions are
+ * granted.
+ */
+final class AuthenticationException extends \RuntimeException implements ZammadException
+{
+}

src/Exceptions/NetworkException.php

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+<?php
+
+declare(strict_types=1);
+
+namespace ZammadAPIClient\Exceptions;
+
+/**
+ * Thrown when a transport-level error prevents the request from completing.
+ *
+ * Unlike HTTP error status codes (which are caught and mapped to typed
+ * exceptions in {@see \ZammadAPIClient\Core\RequestHandler::dispatch()}),
+ * this exception covers failures that occur before or after an HTTP response
+ * is received:
+ *  - DNS resolution failure.
+ *  - TLS handshake error or certificate verification failure.
+ *  - Connection refused or timed out.
+ *  - Unexpected HTTP status codes that do not map to a specific exception
+ *    (i.e. 3xx without redirect following, or unrecognised 4xx).
+ *
+ * The original PSR-18 `ClientExceptionInterface` is preserved as the
+ * `$previous` exception and can be accessed via {@see self::getPrevious()}.
+ */
+final class NetworkException extends \RuntimeException implements ZammadException
+{
+}

src/Exceptions/NotFoundException.php

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+<?php
+
+declare(strict_types=1);
+
+namespace ZammadAPIClient\Exceptions;
+
+/**
+ * Thrown when the Zammad API returns HTTP 404 Not Found.
+ *
+ * Indicates that no resource with the requested ID exists in Zammad.
+ * Possible causes:
+ *  - The resource was deleted (tickets/users can be deleted via the API or UI).
+ *  - The ID is correct but the authenticated user lacks visibility due to group
+ *    or permission restrictions (Zammad returns 404 for permission-hidden resources
+ *    to avoid information leakage).
+ *  - A typo in the resource path or ID.
+ *
+ * The exception message includes the URI of the failing request for debugging.
+ */
+final class NotFoundException extends \RuntimeException implements ZammadException
+{
+}

src/Exceptions/RateLimitException.php

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+<?php
+
+declare(strict_types=1);
+
+namespace ZammadAPIClient\Exceptions;
+
+/**
+ * Thrown when Zammad's rate limit is exceeded and all automatic retries fail.
+ *
+ * {@see \ZammadAPIClient\Core\RetryAfterMiddleware} intercepts HTTP 429
+ * responses and retries the request automatically up to the configured
+ * `$maxRetries` limit, honouring the `Retry-After` response header.
+ * This exception is only raised when all retry attempts are exhausted
+ * and the server still returns 429.
+ *
+ * The {@see self::$retryAfterSeconds} property reflects the last
+ * `Retry-After` value received, giving the caller the opportunity to
+ * implement custom back-off logic or queue the request for later execution.
+ *
+ * The HTTP status code (429) is set as the exception code.
+ */
+final class RateLimitException extends \RuntimeException implements ZammadException
+{
+    /**
+     * @param string $message             Human-readable rate-limit message.
+     * @param int    $retryAfterSeconds   Seconds to wait before retrying, from the last Retry-After header.
+     */
+    public function __construct(
+        string $message,
+        public readonly int $retryAfterSeconds,
+    ) {
+        parent::__construct($message, 429);
+    }
+}

src/Exceptions/ServerErrorException.php

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+<?php
+
+declare(strict_types=1);
+
+namespace ZammadAPIClient\Exceptions;
+
+/**
+ * Thrown when the Zammad API returns an HTTP 5xx status code.
+ *
+ * A 5xx response means the server received the request but encountered an
+ * unexpected internal error processing it. The client request itself was
+ * technically valid; the problem is on the server side.
+ *
+ * Common causes:
+ *  - Zammad application bug or unhandled exception.
+ *  - Underlying database or background-job failure.
+ *  - Zammad server is in maintenance mode or being restarted.
+ *
+ * The HTTP status code is included in the exception message (e.g. "Server error: 503").
+ * No automatic retry is performed for 5xx errors — only HTTP 429 is retried
+ * by {@see \ZammadAPIClient\Core\RetryAfterMiddleware}.
+ */
+final class ServerErrorException extends \RuntimeException implements ZammadException
+{
+}

src/Exceptions/ValidationException.php

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+<?php
+
+declare(strict_types=1);
+
+namespace ZammadAPIClient\Exceptions;
+
+/**
+ * Thrown when the Zammad API returns HTTP 422 Unprocessable Entity.
+ *
+ * Indicates that the request payload was syntactically valid (parseable JSON)
+ * but failed Zammad's business-logic validation. The response body carries a
+ * human-readable `error` string and an optional `details` map; both are
+ * preserved for the caller to inspect or display.
+ *
+ * Common causes:
+ *  - A required field is missing (e.g. no `title` on a ticket).
+ *  - A field value violates a constraint (e.g. unknown state name).
+ *  - A referenced relation does not exist (e.g. non-existent group ID).
+ *
+ * The HTTP status code (422) is set as the exception code.
+ */
+final class ValidationException extends \RuntimeException implements ZammadException
+{
+    /**
+     * @param string                   $message Human-readable error summary from the `error` field.
+     * @param array<int|string, mixed> $errors  Detailed per-field validation messages from `details`.
+     */
+    public function __construct(
+        string $message,
+        public readonly array $errors = [],
+    ) {
+        parent::__construct($message, 422);
+    }
+}

src/Exceptions/ZammadException.php

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+<?php
+
+declare(strict_types=1);
+
+namespace ZammadAPIClient\Exceptions;
+
+/**
+ * Marker interface for all exceptions thrown by the Zammad API client.
+ *
+ * Catching `ZammadException` allows callers to handle any client-specific
+ * error without knowing the concrete subtype. All concrete exceptions extend
+ * `RuntimeException` and implement this interface, so they can be caught as
+ * either `\RuntimeException` or `ZammadException`.
+ *
+ * Exception hierarchy:
+ *  - {@see AuthenticationException} — HTTP 401: invalid token or insufficient permissions.
+ *  - {@see NotFoundException}       — HTTP 404: the requested resource does not exist.
+ *  - {@see ValidationException}     — HTTP 422: the request payload was rejected by the API.
+ *  - {@see RateLimitException}      — HTTP 429: rate limit reached (after all retries exhausted).
+ *  - {@see ServerErrorException}    — HTTP 5xx: unexpected error on the Zammad server side.
+ *  - {@see NetworkException}        — transport-level failure (DNS, TLS, connection refused, etc.).
+ */
+interface ZammadException extends \Throwable
+{
+}