Add ASGI support for TraceErrorsMiddleware (crash reporting)

[amol-]

Goal

Provide an ASGI counterpart to TraceErrorsMiddleware so ASGI applications (Starlette, FastAPI, bare ASGI apps) can get crash reporting through the existing reporters (email, Sentry).

Done criteria:

• A new AsgiTraceErrorsMiddleware (e.g. in backlash/tracing/errors/) that:
  • Passes through non-http scopes (websocket, lifespan) untouched.
  • Catches unhandled exceptions, builds the context (context injectors receive the ASGI scope), captures the traceback, logs it, and dispatches it to the configured reporters.
  • Sends a plain 500 Internal Server Error response when http.response.start has not been sent; logs and re-raises when the response already started (matching AsgiDebuggedApplication post-start semantics from Add ASGI support for the interactive debugger #23).
  • Supports the recorded-exception protocol: after the app completes, a backlash.exc_info key stashed in the scope by the framework is popped and reported (ASGI analog of the WSGI environ['backlash.exc_info'] handshake used by TurboGears).
• Reporters are reused unchanged.
• Error output goes to the backlash logger / sys.stderr (no wsgi.errors equivalent exists in ASGI).
• Behavior is covered by in-process ASGI tests mirroring the existing WSGI middleware semantics.

Out of scope: TraceSlowRequestsMiddleware. Its thread-ident stack snapshotting (sys._current_frames()) is meaningless under an event loop and needs a redesign around asyncio.Task.get_stack(); that deserves its own issue.

Why

#23 brings the interactive debugger to ASGI, but production crash reporting (email/Sentry) remains WSGI-only, so ASGI users get development-time parity without the production-facing half. The middleware is structurally a stripped-down AsgiDebuggedApplication (run app, capture, report, plain 500), and the reporters already consume only the transport-agnostic Traceback object, so the port is a thin adapter (~60 lines) over existing scaffolding.

One caveat worth addressing during implementation: EmailReporter uses synchronous smtplib, which blocks the event loop while reporting. Consider offloading reporter dispatch (e.g. asyncio.to_thread) or documenting the trade-off; the Sentry reporter queues internally and is unaffected.

References

• backlash/tracing/errors/middleware.py (TraceErrorsMiddleware) — WSGI logic to mirror, including the backlash.exc_info / backlash.exc_environ recorded-exception handshake.
• backlash/asgi.py (AsgiDebuggedApplication, from Add ASGI support for the interactive debugger #23) — established ASGI shell patterns: scope filtering, response-started tracking, post-start re-raise, logger/stderr error sink.
• backlash/tracing/reporters/ (mail.py, sentry.py) — transport-agnostic reporters to reuse.
• Issue: Add ASGI support for the interactive debugger Add ASGI support for the interactive debugger #23 — parent effort; this middleware depends on conventions introduced there.