Add in-process HTTP integration tests for webhook-to-metrics flows

[darthfork]

Parent issue: #52

Summary

Add an integration test suite that boots the real promgithub HTTP server in-process and validates behavior through real HTTP requests and /metrics scrapes.

Scope

• Start the real router/server with an isolated Prometheus registry.
• Send signed webhook requests to /webhook.
• Scrape /metrics and assert exported samples.
• Cover /health and invalid-signature behavior.

Scenarios

• workflow_run webhook updates metrics correctly.
• workflow_job webhook updates metrics correctly.
• push webhook updates metrics correctly.
• pull_request webhook updates metrics correctly.
• Invalid signature returns 401 and does not mutate metrics.
• Unsupported event type does not crash and does not emit incorrect metrics.

Design notes

• Prefer scrape-level assertions over direct internal counter inspection.
• Reuse stable JSON fixtures under test_data/ where possible.
• Keep the suite self-contained and fast enough for normal PR execution.

Acceptance criteria

• Tests exercise HTTP request -> handler -> metric scrape end to end.
• Assertions validate real exported metrics, not just internal state.
• The suite runs reliably in CI without external dependencies.