Revert empty context for ASGI runs

[Kludex]

Summary

Reverts #2742 and #2859, which made each ASGI request run in a fresh contextvars.Context to work around python/cpython#140947.

The workaround broke valid use cases that the original discussion did not anticipate:

• Context set in the lifespan or by external instrumentation (e.g. OpenTelemetry eBPF context propagation) is hidden from ASGI handlers. See the comments on explicitly start ASGI run with empty context #2742 from @marctc and @tubarao312.
• The underlying issue is an asyncio bug, not a uvicorn bug. gh-140947: fix contextvars handling for server tasks in asyncio  python/cpython#141158 is expected to fix it upstream for Python >=3.15.

A follow-up PR will reintroduce the behavior as an opt-in flag so users who rely on the isolation can still get it.

Test plan

• uv run ruff check .
• uv run ruff format --check .
• uv run mypy uvicorn
• uv run pytest tests/test_server.py tests/protocols/test_http.py

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

📖 Docs preview: https://8405f61d.uvicorn.pages.dev

[codspeed-hq]

Merging this PR will degrade performance by 11.23%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 23 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_bench_fragmented_body[httptools]	2.2 ms	2.4 ms	-11.23%

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_{Comparing revert-2742 (551589f) with main (850d926)}