#3613 docs: add a middleware tutorial that refuses tool calls

Fixes #3272.

The Middleware page lists "Refuse" among the things a middleware can do, but the only code for it was on the Subscriptions page. This adds a worked example to the Middleware page itself: a middleware that caps how many tool calls run at once and refuses the rest.

It covers the gap #3272 pointed out with a small deterministic example in the middleware docs, rather than the separate model-backed example server the issue proposed.

What it adds

• docs_src/middleware/tutorial002.py
  • max_concurrent_tool_calls(limit), a factory that returns the middleware, on the low-level Server like the page's first example.
  • Only tools/call is counted, so discovery and listing keep answering while tool calls are refused.
  • Over the limit it raises MCPError with the server's own SERVER_BUSY code. MCP has no standard code for "busy", and the spec leaves codes outside the JSON-RPC reserved range to applications.
• docs/advanced/middleware.md
  • A new "A concurrency cap" section after the timing example.
  • The "Refuse" bullet now points at it, and still points at the subscriptions example.
• tests/docs_src/test_middleware.py
  • The call over the limit is refused while the earlier ones are still running, including when it comes from a second client.
  • tools/list is answered with the cap reached.
  • A call that finishes, raises or is cancelled gives its slot back.
  • Answering with an is_error=True result instead of raising reaches the client as a tool result.

Notes

• Nothing changes under src/ and there is no new API.
• The translated pages aren't regenerated here. They show the "behind the English page" notice until the next translation refresh, the same as after any other change to an English page.

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