fix(mcp): isolate structured tool output per request

[cdayAI]

Motivation

• The server stored action-tool structured output in a mutable MCPServer instance field which is shared across concurrent HTTP requests, allowing a race where one request can read or overwrite another request's structuredContent (leak/wrong-goal responses).
• The HTTP transport dispatches requests concurrently with asyncio.to_thread() against a single MCPServer instance, so the per-call structured value must be request-local to be safe.

Description

• Replace the instance-level _structured_override with a request-local ContextVar named _STRUCTURED_OVERRIDE so structured output is isolated to the current call. (_STRUCTURED_OVERRIDE: ContextVar[...]) in packages/maverick-mcp/maverick_mcp/server.py.
• Update handle_tools_call to set the ContextVar to None at call start, use the token to reset() it in a finally block, and read the request-local value with _STRUCTURED_OVERRIDE.get() when assembling structuredContent.
• Change maverick_start and maverick_resume handlers to write their structured result with _STRUCTURED_OVERRIDE.set({...}) instead of writing self._structured_override.
• Add a concurrent regression test test_structured_override_is_request_local in packages/maverick-mcp/tests/test_server.py which simulates two overlapping maverick_start calls and asserts each call receives matching text and structuredContent.

Testing

• Ran style checks with ruff check and applied fixes; linter passed on the modified files.
• Ran unit tests via PYTHONPATH=packages/maverick-mcp:packages/maverick-core:packages/maverick-shield pytest packages/maverick-mcp/tests/test_server.py and all tests passed: 27 passed.
• An initial pytest packages/maverick-mcp/tests/test_server.py run failed in this environment due to import path resolution, after setting PYTHONPATH the full test suite passed.

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Codex Task