Publish typed MCP structured output schemas

[Widthdom]

Summary

• Publish a bounded draft 2020-12 outputSchema for every full MCP tools/list definition, covering all 24 structured tools.
• Require per-tool tool discriminators on success and typed-error payloads, including direct calls, batch-query children, and projected status results.
• Model search recipes, empty excerpts, dependency graphs, partial/truncated results, and finite-depth compatibility values while rejecting incomplete and cross-tool payloads.
• Fail catalog construction when a newly registered structured tool has no output contract.

Why

MCP clients could inspect input schemas but had no machine-readable contract for structured tool results. This made success, partial, and typed-error payloads difficult to validate or generate against, and permissive compatibility fields could hide incomplete or mismatched tool responses.

User impact

Clients can now validate full-catalog MCP structured results against tool-specific schemas. Compact catalogs remain lightweight and direct clients to the full definitions.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --no-build --no-restore --filter 'FullyQualifiedName~ToolsList|FullyQualifiedName~OutputSchema' — 51 passed on net8.0 and 51 passed on net9.0.
• Focused discriminator, exact-byte-budget, batch truncation, and actual-schema tests — 4 passed on each target framework.
• Full-suite run before the final fixes: net8.0 had 10,975 passed / 7 skipped / 4 failed; net9.0 had 10,501 passed / 420 skipped / 4 failed. The three affected regressions per framework and the two distinct existing timing-race failures were rerun after the fixes and all passed on both target frameworks.
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore — succeeded with 0 warnings and 0 errors.
• make lint — passed.
• dotnet tools/CodeIndex.Changelog/bin/Release/net8.0/CodeIndex.Changelog.dll check — validated 74 fragments.
• git diff --check — passed.

Documentation and changelog

• Updated the English and Japanese MCP contract sections in README.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added/updated changelog.d/unreleased/4898.added.md.

Follow-up candidates

None.

Fixes #4898