fix(mcp): keep anticipated tool failures readable under mcp 2.1

[jimisola]

main is red. Its HEAD is the merge commit of #450 (mcp 2.0.0 → 2.1.0), whose build failed — one of the three PRs that landed without waiting for CI in reqstool/.github#111.

Cause

mcp 2.1 narrowed which exceptions reach the client. From the SDK's own docstring:

Any other exception bar MCPError (a protocol error) is treated as a crash: the model sees only Error executing tool <name>, and the server logs the traceback at ERROR.

Only a ToolError keeps its message. The server raised ValueError/RuntimeError for failures it fully anticipated — "Requirement X not found", "sources changed but reloading them failed" — so every one of them was masked. An agent asking for a mistyped REQ_TYPO got Error executing tool get_requirement and no way to see the typo.

This fix raises ToolError at the eight anticipated-failure sites. As a side effect they also drop from ERROR-with-traceback to INFO in the server log, which is where an anticipated failure belongs.

Nothing else changes: a snapshot that cannot be reloaded is still an error and never a fallback to superseded data (bug #437), and startup failures stay RuntimeError because they abort the server rather than answer a tool call.

The regression was six times wider than the test that caught it

Reverting only server.py and keeping the tests fails 8 tests, spanning get_requirement, get_svc, get_mvr, get_requirement_status and get_status. main caught it with one.

The other seven passed on main because they asserted result.is_error and stopped there — true, but blind to whether the model can tell why the call failed. They now assert the message survives too, so a future SDK bump cannot re-mask every error message behind a still-green suite.

Verification

• 987 passed, 2 skipped (main is 986 passed, 1 failed); black --check and flake8 src tests clean.
• Fix reverted, tests kept → 8 failures; fix restored → green. That is what shows the tests actually bind the behaviour rather than describing it.
• CLI output unchanged — the change touches only MCP tool error paths — spot-checked with status against test_basic/baseline/ms-101.
• mcp 2.1.1 carries the identical branch in tools/base.py, so this also unblocks fix(pypi): update dependency mcp to v2.1.1 #451, which is currently sitting open and red for the same reason.

Note

#451 being open-and-red is the reqstool/.github#111 fix working: autoMergeRequest is now None, renovate-approve[bot] still supplies the approving review, and Renovate is holding the merge because build is failing. Its three predecessors merged in 4–28 seconds with red builds.

Refs reqstool/.github#111