Run StreamableHTTP transport tests in process instead of over sockets

[maxisbey]

Final installment of the in-process test migration (#2764, #2765): tests/shared/test_streamable_http.py was the last file spawning uvicorn subprocesses on bind-then-close ports, the pattern that races under pytest-xdist when two workers pick the same ephemeral port. Two of its tests have flaked exactly this way under parallel load (test_server_validates_protocol_version_header, test_streamable_http_client_mcp_headers_override_defaults).

Closes #2704.

Motivation and Context

Same mechanism and fix as the previous two PRs. All four subprocess servers (basic, JSON-response, event-store, context) become in-process apps driven through StreamingASGITransport; with this, no test in the repo binds a port outside the one pre-bound websocket smoke test.

What changed beyond the plumbing swap:

• The 19 raw-HTTP validation tests were synchronous requests-library tests; they're now anyio + httpx over the bridge with the same methods, headers, bodies, and assertions (including suppressing the library-default Accept header where the old tests did).
• wait_for_server is deleted from tests/test_helpers.py — zero users remain. run_uvicorn_in_thread stays for tests/shared/test_ws.py.
• Three # pragma: no cover in src/mcp/server/streamable_http.py are removed (close_standalone_sse_stream, its session-message callback, the JSON-mode Accept rejection): those lines were only ever executed inside the untraced subprocess; the migrated tests now cover them, and keeping the pragmas would fail strict-no-cover.
• One stale src docstring note is dropped: close_standalone_sse_stream claimed client reconnection for standalone GET streams "is NOT implemented — this is a known gap", but the note was self-contradictory from the day it landed — the same commit (Add SSE polling support (SEP-1699) #1654) implemented the auto-reconnect and the test that proves it.
• The long_running_with_checkpoints tool and the slow:// resource branch had no callers anywhere; they're deleted, which moves the tools/list count assertions from 10 to 9 in five tests. Unreachable handler branches elsewhere become dispatch asserts, since they now fail branch coverage instead of hiding in an untraced subprocess.
• test_streamable_http_client_resumption's flag-polling loop (while not flag: sleep(0.1), unbounded) is now two anyio.Events awaited under fail_after(5).
• test_get_sse_stream's 409 assertion is deterministic by construction (the first GET is held open across the second), where the old version's comment admitted it could race.

Deliberately retained: the genuinely time-based tests (SSE polling driven by retry_interval=500, the elapsed >= 0.4 retry-interval assertion, tool sleeps that create the disconnect windows) keep their real-clock behaviour and byte-identical assertions — in-process execution strictly tightens their margins versus the subprocess version. A follow-up may convert the reconnection choreography to event-store-sequenced waits (retry_interval=0 + the interaction suite's wait_until_stored pattern) and shave most of the file's remaining ~6s runtime; that's a behaviour-preserving quality pass kept out of this PR so the harness swap stays auditable.

How Has This Been Tested?

• ./scripts/test green: 1526 passed, 100% line+branch coverage including tests/, strict-no-cover clean
• The file 3× solo and together with the four previously-migrated files under pytest-xdist -n 4, 3× — stable; the nine resumption/reconnection tests 5× solo
• Targeted coverage runs confirm the three de-pragma'd src lines (and both arcs of the branchy one) are executed by this file alone

Breaking Changes

None — test-only, plus pragma/docstring removals in src.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Several tests here overlap with tests/interaction/transports/ coverage (resumption-token replay, stream-close auto-reconnect, priming events, standalone GET delivery); deduplication is inventoried for a follow-up ticket, kept out of scope so this diff remains a pure harness swap.

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