fix(mcp): remove top-level anyOf from check_index_coverage schema

[DaveMurray-AIGuys]

Fixes #1524

Problem

check_index_coverage's inputSchema had a top-level anyOf:

"required":["project"],"anyOf":[{"required":["paths"]},{"required":["scopes"]}]

Anthropic (Claude), AWS Bedrock, Google Vertex/Gemini, and Azure OpenAI reject custom tool schemas containing a top-level oneOf/allOf/anyOf. Once this tool's schema reaches the model (e.g. after tools/list), the provider rejects the request outright, breaking every tool call in the session for MCP clients backed by those providers.

Fix

• Removed the top-level anyOf from check_index_coverage's inputSchema.
• The "at least one of paths or scopes" requirement was already enforced independently at runtime in handle_check_index_coverage (path_count == 0U && scope_count == 0U rejection), so no validation is lost.
• Moved the requirement into the tool description and the paths/scopes property descriptions so it's still discoverable by callers.
• No other tool schema in src/mcp/mcp.c uses oneOf/allOf/anyOf (verified via grep), so this was the only occurrence of the defect.

Testing

• make -f Makefile.cbm cbm — clean build.
• make -f Makefile.cbm test-focused TEST_SUITES=mcp (ASan+UBSan) — 192 passed, 2 skipped (Windows-only), 0 failed, including all 6 tool_check_index_coverage_* tests exercising paths/scopes/error paths.
• Manually verified via stdio JSON-RPC (tools/list) that the live inputSchema for check_index_coverage no longer contains anyOf, only "required":["project"].
• scripts/check-dco.sh origin/main..HEAD passes.

This is a one-file, no-behavior-change fix (schema only); scope is limited to the bug fix per the "bug fixes ... welcome without prior discussion" exception in CONTRIBUTING.md, and #1524 documents the root cause and repro for maintainer review.

[github-actions]

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