feat(testing): build_test_client(platform) async context manager

[bokelley]

Closes #549

Summary

Adds build_test_client(platform) to adcp.testing — a single async with block that replaces the 4-line boilerplate every in-process integration test previously needed:

# Before
app = build_asgi_app(platform)
async with LifespanManager(app):
    async with httpx.AsyncClient(transport=httpx.ASGITransport(app=app), base_url="http://test") as client:
        resp = await client.post("/mcp/", json=...)

# After
async with build_test_client(platform) as client:
    resp = await client.post("/mcp/", json=...)

Also adds allowed_hosts: Sequence[str] | None = None to build_asgi_app so callers can configure FastMCP's transport-security layer directly. build_test_client sets this automatically from the base_url hostname.

Key implementation notes:

• asgi_lifespan is lazy-imported with an actionable ImportError message (pip install 'adcp[dev]')
• build_asgi_app runs inside asyncio.to_thread() because validate_capabilities_response_shape calls asyncio.run() internally, which fails if a loop is already running in the calling async test
• follow_redirects=True default matches all existing ASGI test fixtures (FastMCP can issue a 307 on /mcp → /mcp/)
• headers= accepted as Mapping[str, str] | None and forwarded directly to httpx.AsyncClient

Nits surfaced in pre-PR review (not fixed — low priority):

• asgi-lifespan lives in [dev] rather than a [test] extra; the ImportError message will need updating if the extra is renamed

What was tested

• pytest tests/test_testing_decisioning.py — 20 passed (12 existing + 8 new)
• ruff check src/adcp/testing/ tests/test_testing_decisioning.py — clean
• Includes end-to-end test: test_build_test_client_can_make_request makes a real MCP initialize call through the in-process ASGI stack and asserts HTTP 200
• Includes ImportError test: patches asgi_lifespan out of sys.modules and asserts the error message fires

Pre-PR review:

• code-reviewer: approved — no blockers; all raised issues fixed before push
• dx-expert: approved — asyncio.to_thread pattern is correct, DX score 5/5 on time-to-hello-world

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01UZyqrAkoJPCEz5VLerzxFv

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