fix(host-agent): align MCP integration with mcp SDK 1.28.1 realities

Three final-review deviations closed:

I1 (session-end release): mcp SDK 1.28.1 exposes no per-session
shutdown callback (only a server-level lifespan). Lower the
McpBusyTracker default TTL from 60s to 20s and update spec §6.5,
Q5/R3, D9, and docs/MCP_INTEGRATION.md concurrency section to
document the TTL-only recovery path. 20s is short enough to recover
within one 30s heartbeat interval but long enough that an active
session does not lose its lease during normal operator pauses.

I2 (JSON-RPC error shape): FastMCP Tool.run wraps every non-
UrlElicitationRequiredError exception (including McpError with typed
ErrorData) into ToolError, which the lowlevel call_tool handler
serializes as CallToolResult(isError=true, content=[TextContent(...)]).
There is no public path that surfaces JSON-RPC -32000 with structured
data.busy_owner from a tool call site. Update spec §7 error matrix
and docs/MCP_INTEGRATION.md error table to document the actual wire
shape; busy_owner now lives in the text content.

I3 (typing): mcp_server: Any = None -> FastMCP | None = None via
TYPE_CHECKING, keeping the mcp import lazy (matches precedent
elsewhere in the codebase) while adding static type checking at the
create_console_app boundary.

Tests added (4):
- test_default_ttl_is_20_seconds — locks I1's new default TTL
- test_default_ttl_recovers_dead_session_within_one_window — locks
  I1's recovery semantics (lease sweeped on next read after 20s)
- test_busy_error_wire_shape_is_calltoolresult_iserror — pins I2's
  wire envelope via Tool.run + lowlevel Server._make_error_result
- test_busy_error_text_includes_cloud_assignment_owner — same for
  the cloud_assignment busy_owner branch

Full non-integration suite: 697 passed / 54 deselected (was 693 / 54).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/device-host-agent/host_agent/web/app.py

@@ -5,7 +5,7 @@ import base64
 import json
 from collections.abc import Awaitable, Callable
 from pathlib import Path
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
 import jinja2
 from fastapi import Depends, FastAPI, HTTPException, Request
@@ -35,6 +35,9 @@ from host_agent.web.auth import (
 )
 from host_agent.web.mcp_auth import BearerAuthMiddleware
 from storage.device_config import DeviceConfigStore
+
+if TYPE_CHECKING:
+    from mcp.server.fastmcp import FastMCP
 from storage.task_metadata import TaskMetadataStore
 from storage.timeline import Timeline
 
@@ -226,7 +229,7 @@ def create_console_app(
     metadata_store: TaskMetadataStore | None = None,
     timeline: Timeline | None = None,
     executor: AssignmentExecutor | None = None,
-    mcp_server: Any = None,
+    mcp_server: FastMCP | None = None,
     mcp_token_store: McpTokenStore | None = None,
     mcp_busy_tracker: McpBusyTracker | None = None,
 ) -> FastAPI: