Support multiple validators, add "Using Validators" section

.agents/tasks/archive/rewrite-external-messages-section.md

@@ -0,0 +1,22 @@
+# Task: Rewrite the "External messages" section of the documentation
+- New name for the section: "Using Validators"
+- Purpose: explain how to use validators in code, including how to create and apply them to proto messages.
+- Target outcome: a reader can create a validator for a proto message, apply it to an external
+  message and to a local message likewise.
+- Cases for using validators
+  - External messages: the message the modification of source code is not possible (e.g. messages in libraries).
+  - Validation of local messages that require computation or custom logic that cannot be expressed in proto options.
+- Describe that multiple validations can be applied to the same message type.
+
+## Describe creating a validator
+- Implement `io.spine.validation.MessageValidator<M>`.
+- Describe automatic discovered of validators using the `ServiceLoader` mechanism.
+- Mention using `AutoService` to generate the service provider configuration file.
+- Mention that it is possible to add a validator to `ValidatorRegistry` explicitly during
+  application startup or configuration. This is an alternative to service discovery approach.
+- Describe the requirement for a public no-args constructor.
+
+## Keep the `implement-an-external-validator.md` page
+- Rename the file to `implement-a-validator.md`.
+- Remove using the `@Validator` annotation from the example, and instead show using `@AutoService`.
+- Add the page to `sidenav.yml`.

.agents/tasks/third-party-messages-plan.md → .agents/tasks/archive/third-party-messages-plan.md

@@ -12,7 +12,7 @@ validating local messages.
 ## Placement
 
 - Placement of the page: after “Built-in options”, before “Custom validation”.
-- Hugo section (minimal change): add the page under `docs/content/docs/validation/02-concepts/`.
+- Hugo section (minimal change): add the page under `docs/content/docs/validation/03-built-in-options/`.
   If the site navigation later gains an “Advanced topics” section, the page can move there.
 
 ## Planned content
@@ -39,20 +39,17 @@ validating local messages.
   - Return `List<DetectedViolation>`.
   - Use `FieldViolation` (and other available violation types) to point at a field path and value.
   - Mention that the runtime converts `DetectedViolation` into `ConstraintViolation`/`ValidationError`.
-- Constraints and guardrails:
-  - Exactly one validator per external message type (duplicate is an error).
-  - Validators for local messages are prohibited (use options/codegen instead).
 - Example walkthrough (short, copy-pastable):
-  - Implement `EarphonesValidator` (from `:tests:validator`) and show how it affects a local message
-    that contains an `Earphones` field.
+  - Implement `TimestampValidator` (from `:jvm-runtime`) and show how it affects a local message
+    that contains a `Timestamp` field.
 
 ## Source references to anchor the docs
 
 - External validation API and requirements:
   - `jvm-runtime/src/main/kotlin/io/spine/validation/MessageValidator.kt`
   - `jvm-runtime/src/main/kotlin/io/spine/validation/Validator.kt`
 - Example implementation:
-  - `tests/validator/src/main/kotlin/io/spine/tools/validation/test/EarphonesValidator.kt`
+  - `jvm-runtime/src/main/kotlin/io/spine/validation/TimestampValidator.kt`
 
 ## Output
 

.agents/tasks/archive/using-validator-registry-section.md

@@ -0,0 +1,22 @@
+# Task: create a page about `ValidatorRegistry` and how to use it
+
+## Main idea
+In simple cases the programmer using the Validation library does not need to interact
+with `ValidatorRegistry` directly, as the library discovers and applies validators automatically.
+
+However, in some cases it may be necessary to interact with the registry directly,
+for example, to add a validator explicitly or to query the registry for registered validators.
+
+## Features
+- Discovery and loading validators using the `ServiceLoader` mechanism.
+- Setting `validator` placeholder in the reported violation messages.
+- Adding validators.
+- Quering and removal of registered validators.
+
+## Edge case
+- Overwriting validation by automatically registered validators by removing them from the
+  registry and adding new validators.
+
+## Placement
+- Add the page after "Implement a validator" page.
+- Update `sidenav.yml` accodringly.

.agents/tasks/archive/validator-registry.md

@@ -0,0 +1,25 @@
+# Task: Implement a registry for custom validators of Protobuf messages
+
+This task involves creating a registry system that allows for the dynamic registration and retrieval
+of custom validators for Protobuf messages.
+The registry should support adding new validators at runtime and efficiently look up validators
+based on message types.
+
+## Goals
+- Design a flexible and efficient registry for custom validators implementing
+the `MessageValidator` interface.
+
+## Target class
+- `io.spine.validation.ValidatorRegistry`
+
+## Module
+- `jvm-runtime`
+
+## Features
+- Ability to add custom validators for specific Protobuf message types.
+- Support several validators per type.
+- Ability to remove validators from the registry for a given type.
+- Ability to clear the whole registry.
+- Load validators from the classpath using Java's ServiceLoader mechanism.
+- API for validating a message by looking up its type in the registry and
+  applying all associated validators.

build.gradle.kts

@@ -85,7 +85,6 @@ spinePublishing {
         "java",
         "java-bundle",
         "jvm-runtime",
-        "ksp",
     )
     modulesWithCustomPublishing = setOf(
         "java-bundle",

buildSrc/src/main/kotlin/fat-jar.gradle.kts

@@ -92,8 +92,8 @@ tasks.shadowJar {
         "resources/com/pty4j/**",
 
         // Protobuf files.
-        "google/**",
-        "spine/**",
+        "google/**/*.proto",
+        "spine/**/*.proto",
         "src/**",
 
         // Java source code files of the package `org.osgi`.

buildSrc/src/main/kotlin/io/spine/dependency/local/Validation.kt

@@ -36,7 +36,7 @@ object Validation {
     /**
      * The version of the Validation library artifacts.
      */
-    const val version = "2.0.0-SNAPSHOT.395"
+    const val version = "2.0.0-SNAPSHOT.400"
 
     /**
      * The last version of Validation compatible with ProtoData.

buildSrc/src/main/kotlin/io/spine/dependency/test/JUnit.kt

@@ -33,7 +33,7 @@ import io.spine.dependency.DependencyWithBom
 @Suppress("unused", "ConstPropertyName")
 object JUnit : DependencyWithBom() {
 
-    override val version = "6.0.0"
+    override val version = "6.0.3"
     override val group: String = "org.junit"
 
     /**