fix: unwind exit stacks with in-flight exception details so generator dependencies can roll back

[JasperSui]

Fixes #255

Problem

When a function decorated with @injectable raises, generator dependencies never see the exception: exit stacks are always closed via aclose() (i.e. __aexit__(None, None, None)), so a dependency's except/rollback branch can never run — unlike a failing FastAPI request, which unwinds its exit stack with the exception details.

There was also a deeper, silent layer to this: on fastapi>=0.121 generator dependencies are entered into the two inner request-scope stacks (fastapi_inner_astack / fastapi_function_astack), and those were registered on the owning stack as push_async_callback(stack.aclose) — stripping exception details even when the owning stack did unwind with them. This meant InjectableScope.__aexit__'s existing exception forwarding silently did nothing for generator dependencies.

Fix

1. main.py — register the inner FastAPI stacks via push_async_exit so exception details reach the stacks where generator teardowns actually live.
2. util.py / async_exit_stack.py — new optional exc= parameter on cleanup_exit_stack_of_func() / cleanup_all_exit_stacks() (plumbed through AsyncExitStackManager): when provided, stacks unwind via __aexit__(type(exc), exc, exc.__traceback__), exactly as if the exception had propagated out of an async with block. A dependency re-raising the in-flight exception (except: rollback(); raise) is recognized as normal context-manager protocol, not reported as a cleanup failure; genuinely distinct teardown failures still surface as DependencyCleanupError.
3. injectable_scope() — with (1) in place, an exception propagating out of the async with block now reaches generator dependencies automatically, giving full FastAPI parity with zero extra plumbing:

async with injectable_scope():
    await process()  # raises -> conn.rollback() + conn.close() run during unwind

Intentional semantic note

As in FastAPI, when an exception is delivered to a generator dependency, code placed after a bare yield (no try/finally) does not run — teardown that must always run belongs in finally. test_exception_inside_scope_still_cleans_up was updated to pin this contract (cleanup still always runs when written with finally).

Pre-existing bugs fixed along the way

• Sync-path loop stickiness (concurrency.py): under the default "current" strategy, once the thread had no policy event loop set (e.g. after any pytest-asyncio test), every run_coroutine_sync call created a fresh throwaway loop without registering it. Dependencies were then resolved on loop A while cleanup ran on loop B, and the per-loop stack registry ([BUG] pytest hangs when using async_get_injected_obj #186) rightly refused to close A's stacks from B — silently skipping generator teardown. Reproducible on main: running any async test file before test_integration.py breaks 12+ sync integration tests. The created loop is now registered via asyncio.set_event_loop (mirroring pre-3.12 get_event_loop auto-create semantics), with a guard for closed policy loops.
• Latest-mypy failure (util.py): return value inside the asyncgen loop now uses cast("T2", value) — the previous type: ignore[no-any-return] fails on the current mypy release (CI installs unpinned mypy).

Verification

• Full suite green on nox 3.10 / 3.13 / 3.14 (230 passed each) and in the dev venv
• nox -s coverage: 100% statements + branches
• nox -s mypy: clean; ruff check + ruff format --check: clean; nox -s docs-build: success
• New end-to-end regression tests (test/test_exit_stack_exception_unwind.py) cover: the issue's exact repro via cleanup_all_exit_stacks(exc=...) and cleanup_exit_stack_of_func(..., exc=...), automatic forwarding via injectable_scope(), sync generator dependencies, the fully-sync entry point, a dependency that swallows the exception, and the unchanged no-exception commit path

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (28f2609) to head (ec6334d).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #259   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          580       595   +15     
  Branches        69        72    +3     
=========================================
+ Hits           580       595   +15     

Files with missing lines	Coverage Δ
src/fastapi_injectable/async_exit_stack.py	100.00% <100.00%> (ø)
src/fastapi_injectable/concurrency.py	100.00% <100.00%> (ø)
src/fastapi_injectable/main.py	100.00% <100.00%> (ø)
src/fastapi_injectable/scope.py	100.00% <ø> (ø)
src/fastapi_injectable/util.py	100.00% <100.00%> (ø)

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