feat: add request context support

[dustinbyrne]

💡 Motivation and Context

Add request-scoped PostHog context helpers so PHP server-side captures, exception events, and feature flag calls can inherit request metadata when framework middleware opts in.

This keeps the SDK framework-neutral while giving Laravel, Symfony, and other integrations a shared core API.

Behavior:

• PostHog::withContext() / Client::withContext() wrap a callback with scoped context.
• Context is isolated per Client and per Fiber, and restored automatically.
• contextFromHeaders() extracts PostHog frontend tracing headers (X-PostHog-Distinct-Id, X-PostHog-Session-Id).
• capture() applies contextual distinct ID, session ID, and properties while preserving explicit capture values.
• Captures without a real distinct ID become personless events with $process_person_profile: false; generated IDs are not used for deprecated send_feature_flags evaluation.
• Exception capture and feature flag helpers can use the current request context.
• Laravel and Symfony examples show request-boundary usage with ['fresh' => true].

💚 How did you test it?

• vendor/bin/phpcs --standard=phpcs.xml lib/RequestContext.php lib/Client.php lib/PostHog.php lib/ExceptionCapture.php test/RequestContextTest.php test/ExceptionCaptureTest.php examples/request-context-laravel-middleware.php examples/request-context-symfony-kernel.php
• vendor/bin/phpunit --no-coverage --filter 'RequestContextTest|ExceptionCaptureTest'
• 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

[dustinbyrne]

This moved into applyCaptureContext