feat: add source-specific typed request input

[memleakd]

Description

This PR proposes source-explicit typed input access for IncomingRequest:

$page = $request->getQueryInput()->integer('page', 1);
$remember = $request->getPostInput()->boolean('remember', false);
$name = $request->getPayloadInput()->string('name');

This builds on CodeIgniter\Input\InputData and gives applications a clearer way to consume raw request values without relying on mixed request input.

The new methods keep the input source visible at the call site:

• getQueryInput() reads query-string parameters.
• getPostInput() reads POST body parameters.
• getPayloadInput() reads the request body payload.

getPayloadInput() follows the same request-source behavior used by Form Requests: JSON requests use the decoded JSON body, PUT, PATCH, and DELETE requests use parsed raw input when they are not multipart requests, and ordinary form requests use POST body parameters. For non-JSON GET and HEAD requests, payload input is empty so query-string reads remain explicit through getQueryInput().

These helpers do not validate input or replace Validation/FormRequest. They only provide fallback-friendly typed access to source-specific raw request data.

Tests cover source isolation, typed fallback access, JSON payloads, raw write-method payloads, POST payloads, empty GET/HEAD payloads, scalar JSON rejection, and invalid JSON behavior.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[memleakd]

The current PHPStan failure appears unrelated to this PR. It is in system/View/Parser.php, which this PR does not touch.