tests: avoid racy HTTP test ports

[tarunag10]

Summary

• Replaces racy pick-free-port-then-rebind server fixtures in the streamable HTTP/SSE test files with the existing run_uvicorn_in_thread helper.
• Updates affected tests to consume URLs yielded from the actual listening uvicorn socket instead of separately selected port integers.
• Adds a regression assertion that the basic streamable HTTP fixture URL uses a port that is already reserved, and closes the helper socket explicitly during shutdown.

This is an alternative/focused fix for #2704 using the existing thread helper. I noticed #2705 is already open with a process-helper approach; this draft is opened for maintainer comparison rather than as a duplicate ready-to-merge submission.

Validation

• uv run ruff check tests/test_helpers.py tests/shared/test_streamable_http.py tests/shared/test_sse.py tests/server/test_sse_security.py tests/server/test_streamable_http_security.py tests/client/test_http_unicode.py
• uv run pytest tests/shared/test_streamable_http.py tests/shared/test_sse.py tests/server/test_sse_security.py tests/server/test_streamable_http_security.py tests/client/test_http_unicode.py -> 120 passed, 1 skipped
• uv run pytest -n auto tests/shared/test_streamable_http.py tests/shared/test_sse.py tests/server/test_sse_security.py tests/server/test_streamable_http_security.py tests/client/test_http_unicode.py -> first run had one non-port timing failure in test_streamable_http_multiple_reconnections; isolated rerun passed; second full affected xdist run passed with 120 passed, 1 skipped