Unbounded ProcessInParallel extension abandons started tasks fire-and-forget on mid-enumeration failure

[thomhurst]

Found in the pre-4.0 audit (follow-up to #360/#361).

AsyncEnumerableExtensions.ProcessInParallel<T, TOutput>(items, selector, maxConcurrency: null, ...) — the unbounded path has no try/finally around enumeration, unlike the equivalent processor class. If MoveNextAsync throws or the token cancels mid-enumeration, tasks already started keep running fire-and-forget and their failures surface only as UnobservedTaskException at GC.

Failure scenario: source yields 50 items then throws; 50 selector tasks are in flight → the extension rethrows the enumeration error immediately, the 50 tasks are abandoned, and any of their exceptions hit TaskScheduler.UnobservedTaskException later.

Same fluent shape via the processor class (SelectAsync(...).ProcessInParallel()) produces a structured wait in its finally; the extension and processor paths should behave identically.

Proposal: wrap enumeration in try/finally that awaits (and observes) started tasks, mirroring AsyncEnumerableParallelProcessor — while coordinating with the exception-masking fix tracked separately.

[thomhurst]

Resolved by #370 (merged as commit 9c4c40f). The unbounded ProcessInParallel extension now drains and observes started tasks after mid-enumeration failure while preserving the source exception. Verified on current main; full dotnet test passes 1,716/1,716 tests across net8.0, net9.0, and net10.0.