feat(client): ADCPClient.from_mcp_client() factory for in-process MCP transport

[bokelley]

Closes #291

Summary

Adds ADCPClient.from_mcp_client(client, *, agent_id=None, ...) — a classmethod that wraps a pre-connected mcp.ClientSession and returns a fully configured ADCPClient. Parity with JS AgentClient.fromMCPClient() (v5.19.0). The primary use case is compliance test fleets that need the full client path exercised against an in-process MCP server via mcp.shared.memory.create_client_server_memory_streams, without standing up a loopback HTTP server.

Mechanics: MCPAdapter._get_session() already short-circuits when self._session is not None. The factory injects the caller-owned session there and sets a _session_is_injected flag so close() skips _cleanup_failed_connection — the caller owns the lifecycle. All existing client surface (validation, idempotency, validate_features, capability cache) works unchanged through the factory path. Request signing is intentionally absent from the factory params: the hook is wired into the HTTP transport layer that is bypassed here.

What was tested

• pytest tests/test_protocols.py::TestFromMcpClientFactory — 8 new tests: session injected, unique default agent_id, explicit agent_id, tool calls route through injected session, close() no-op, async with exit no-op, validation wired, unique idempotency tokens
• pytest tests/ -x -q --ignore=tests/integration --ignore=tests/conformance/signing/test_ip_pinned_transport.py — 2185 passed, 21 skipped, 0 failures
• ruff check src/adcp/protocols/mcp.py src/adcp/client.py — clean
• mypy src/adcp/protocols/mcp.py src/adcp/client.py — clean

Nits surfaced (not fixed — reviewer discretion)

• test_tool_call_routes_through_injected_session calls adapter._call_mcp_tool directly rather than the public get_products method; either level is valid, but the public surface is more refactor-stable
• strict_idempotency=True wires idempotency_capability_check on the adapter; a test asserting the hook is set would complete coverage of that flag
• No test for the uninitialized-session failure mode (callers who forget await session.initialize() get an opaque MCP error string in TaskResult.error)
• testing-guide.md (if present) does not reference from_mcp_client — follow-on documentation task

Bucket gap

No client label exists in this repo. Bucket for this issue is client (ADCPClient surface). Flagged in run summary.

Pre-PR review

• code-reviewer: approved — no blockers; 3 issues resolved in follow-up commits (None guard on agent_id, if not isinstance guard replacing assert, ClientSession type annotation via TYPE_CHECKING)
• dx-expert: approved — no blockers; nit fixes applied (param ordering to match __init__, inline lifecycle comment in docstring example); server-wiring placeholder comment noted as adequate for first iteration

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See adcp#3121
for context.

Session: https://claude.ai/code/session_014Lazxs8FCWcVzzKi8MpLPi

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