fix(tuic): clean up connection child resources

[Itsusinn]

Summary

• cancel per-connection spawned datagram/stream handler tasks when the QUIC connection ends
• cancel and invalidate UDP association sessions during connection teardown so bridge tasks and outbound sockets are released
• add regression coverage for child-task cancellation and UDP session teardown

Tests

• cargo +nightly fmt --all
• cargo test -p wind-tuic
• cargo check -p tuic-server

[codspeed-hq]

Merging this PR will not alter performance

✅ 22 untouched benchmarks
⏩ 11 skipped benchmarks¹

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_{Comparing codex/fix-tuic-resource-leaks (d6bfa0f) with main (4003eae)}

Footnotes

1. 11 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. ↩