test: add MCP protocol smoke test for the stdio transport

[yagna-1]

What

Adds two end-to-end tests in tests/test_mcp_protocol.py that spawn excel-mcp-server stdio as a real subprocess and exercise the JSON-RPC protocol over its stdin/stdout:

• test_initialize_handshake_succeeds — the server completes the MCP initialize handshake and advertises the tools capability.
• test_documented_tools_are_listed — create_workbook and get_workbook_metadata (both in TOOLS.md) are returned by tools/list with valid object input schemas.

Why

The existing test_sandbox_paths.py exercises path-resolution internals directly, but nothing currently runs the actual MCP entry point. A regression in __main__.py, the FastMCP wiring in server.py, or the handshake sequencing could ship without any test failing. These two tests close that gap.

Implementation

• One new file, ~100 LoC, follows the existing unittest style of test_sandbox_paths.py.
• Skipped automatically when pytest-mcp-plugin is not installed or excel-mcp-server is not on PATH, so the suite stays green in any environment, including the current publish.yml workflow which doesn't install dev deps.
• No production code changes, no pyproject.toml deps changes, no workflow changes. You decide whether to wire it in.

To run locally:

```sh
uv add --dev "pytest-mcp-plugin>=0.2.3"
uv sync
uv run pytest tests/test_mcp_protocol.py -v
```

Verified locally

```text
$ uv run pytest tests/ -q
....... [100%]
7 passed in 0.97s
```

That's 5 existing + 2 new. Server runs in stdio mode — no Excel files touched.

Disclosure

I'm the author of pytest-mcp-plugin (MIT, on PyPI). It's a small community library that drives MCP servers over stdio/HTTP from inside the test runner — Apache/MIT-compatible, not a vendored dependency. Repo: https://github.com/yagna-1/mcp-test.

While testing this PR I caught and fixed a bug in pytest-mcp-plugin itself: the client wasn't sending notifications/initialized after the MCP initialize handshake, which FastMCP requires before it will return tools from tools/list. That fix shipped as v0.2.3 (May 5). The pin in this PR is >=0.2.3 so users get the working version.

Made with Cursor

[yagna-1]

Heads-up: pytest-mcp-plugin v0.3.0 was just released. The >=0.2.3 requirement in the docstring still holds (newer versions are backward-compatible for this test); no changes needed here.

For context, v0.3.0 also runs Anthropic's @modelcontextprotocol/conformance suite in CI against its bundled FastMCP demo server with a locked baseline — so if you ever want a "do we pass the official MCP spec?" check for excel-mcp-server, the same machinery is now one CLI invocation away (mcp-test conformance --command "excel-mcp-server stdio").