feat: add request context support

[dustinbyrne]

💡 Motivation and Context

Add request-scoped PostHog context helpers so server-side captures and exception events during a PHP request can inherit useful request metadata and, when enabled by an integration/user middleware, PostHog tracing headers (X-PostHog-Distinct-Id, X-PostHog-Session-Id).

This aligns PHP SDK behavior with the server-side request context guidelines while keeping the implementation framework-neutral because this repo does not include first-party Laravel/Symfony middleware.

Behavior:

• RequestContext provides scoped request context that is restored automatically and isolated across Fibers.
• capture() applies context properties/session/distinct ID in the core capture path, with explicit capture values taking precedence.
• Missing capture identity now generates a personless UUID event via $process_person_profile: false.
• captureException() and automatic exception capture can inherit request context through the core capture path.
• contextFromHeaders() only reads X-PostHog-Distinct-Id / X-PostHog-Session-Id; framework integrations should expose their own use_tracing_headers option and skip this helper when disabled.
• Added a single-file Laravel/Symfony-style example showing how integration middleware can wrap the request while keeping use_tracing_headers integration-scoped.

💚 How did you test it?

• php -l examples/request-context-frameworks.php
• ./vendor/bin/phpunit --no-coverage test/RequestContextTest.php
• ./vendor/bin/phpcs --standard=phpcs.xml lib/RequestContext.php lib/Client.php lib/PostHog.php test/RequestContextTest.php
• ./vendor/bin/phpunit --no-coverage (passes with existing warnings/deprecations)

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file