feat: add Rails request context support

[dustinbyrne]

💡 Motivation and Context

Add Rails request-scoped PostHog context so server-side captures and exception events during a request can inherit useful request metadata and, when enabled, PostHog tracing headers (X-PostHog-Distinct-Id, X-PostHog-Session-Id).

This aligns Ruby/Rails behavior with Node/Python server SDK context handling while keeping the context implementation internal to avoid expanding the public Ruby API surface.

Middleware behavior:

• RequestContext now always wraps Rails requests and applies request metadata ($current_url, $request_method, $request_path, $user_agent, $ip) to captures made during the request.
• PostHog tracing header identity/session usage is controlled separately via config.use_tracing_headers.
• CaptureExceptions only extracts authenticated Rails user identity and exception-specific metadata; request context identity/session/properties are applied by the core capture path.

💚 How did you test it?

• bundle exec rspec spec/posthog/client_spec.rb spec/posthog/rails/request_context_spec.rb spec/posthog/rails/railtie_spec.rb
• bundle exec rubocop --force-exclusion lib/posthog/client.rb posthog-rails/lib/posthog/rails/configuration.rb posthog-rails/lib/posthog/rails/request_context.rb posthog-rails/lib/posthog/rails/request_metadata.rb posthog-rails/lib/posthog/rails/capture_exceptions.rb spec/posthog/rails/request_context_spec.rb
• Validated behavior against local Node SDK context tests: pnpm test:unit src/__tests__/context.spec.ts --runInBand
• Validated Python context/capture behavior with targeted pytest coverage for contexts and capture exception paths

📝 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