Ensure mounted transports are closed even if main transport raises

[mbeijen]

If a client with proxy mounts fails to close the main transport (its close()/exit/aclose()/aexit raises), the mounted transports were never reached, leaking their connections. Wrap the main transport cleanup in try/finally so mounts are always closed regardless.

Ported from encode/httpx#3769.

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing mbeijen:mounted-transports-close-on-raise (d543093) with main (c4e73cf)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}