Close the consumed subscription source iterator

[AryanRG]

Fixes #283.

Close the iterator returned by aiter(iterable), rather than the iterable wrapper, so subscription cleanup reaches the source actually being consumed.

Adds regression tests for explicit close, thrown errors and callback errors, plus a test through subscribe().

Validation: full suites on Python 3.10 and 3.14, required coverage, Ruff and mypy.

[codspeed]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing AryanRG:fix-close-subscription-iterator-283 (5d1ab0b) with main (d913a61)}

[Cito]

Thank you, this looks like the right way to fix it.

I would only change two minor things, but will do it after merging.