Add v3 breaking changes notice to README

[jlowin]

Adds a notice at the top of the README advising users to pin fastmcp<3 to avoid unexpected issues as FastMCP 3.0 development proceeds.

[marvin-context-protocol]

Test Failure Analysis

Summary: The Windows test suite timed out on test_multi_client in tests/test_mcp_config.py.

Root Cause: This PR branch (readme-v3-pin-notice) doesn't include the fix from PR #2711 which was merged to main just before this workflow run. The test_multi_client test spawns Python subprocesses and performs stdio communication, which consistently times out on Windows CI due to slower subprocess performance.

Suggested Solution: Merge or rebase this PR branch with the latest main branch to pick up the recently merged fix. However, note that PR #2711 only added a skip marker for test_multi_client_parallel_calls, not for test_multi_client.

The test_multi_client test (lines 204-244) is similar to test_multi_client_parallel_calls but doesn't have the Windows skip marker. Given that it's also timing out on Windows for the same reasons (subprocess lifecycle issues), it should probably also be skipped on Windows.

Action items:

1. Rebase this PR on latest main
2. Consider adding a Windows skip marker to test_multi_client following the pattern from test_multi_client_parallel_calls and test_multi_client_lifespan

Detailed Analysis

The test failure occurred at line ~204 in tests/test_mcp_config.py:

async def test_multi_client(tmp_path: Path):
    # Creates 2 FastMCP server subprocesses
    # Calls list_tools() and call_tool() via stdio
    # Times out on Windows due to subprocess overhead

Similar tests in the same file already skip Windows:

• test_multi_client_parallel_calls (line 250) - has @pytest.mark.skipif(sys.platform.startswith("win32"))
• test_multi_client_lifespan (line 299) - has the same skip marker
• test_multi_client_force_close (line 364) - also skips Windows

The pattern is clear: tests that spawn Python subprocesses with stdio communication are problematic on Windows CI.

Relevant log excerpt:

tests\test_mcp_config.py ........+++++++++++++++++++++++++++++++++++ Timeout +++++++++++++++++++++++++++++++++++

The 8 dots indicate 8 passing tests before the timeout on the 9th test (test_multi_client).

Related Files

• tests/test_mcp_config.py:204-244 - The test_multi_client test that timed out
• tests/test_mcp_config.py:250-292 - The test_multi_client_parallel_calls test that already has a Windows skip marker (from PR Skip parallel MCP config test on Windows #2711)