test(mcp): reject narrowing optional additions

[barbatdev]

🔗 Linked Issue

Upstream issue: Gentleman-Programming#717 (status:approved).

This is a fork-first corrective PR and intentionally does not close the upstream issue.

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🏷️ PR Type

• type:bug — Bug fix
• type:feature — New feature
• type:docs — Documentation only
• type:refactor — Code refactoring (no behavior change)
• type:chore — Maintenance, dependencies, tooling
• type:breaking-change — Breaking change

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📝 Summary

• Reject optional named property additions when v1 allowed arbitrary additional-property values.
• Preserve compatibility for simple optional additions when v1 previously rejected unknown properties.
• Add the focused regression required by native RDD finding R3-001.

📂 Changes

File	Change
internal/mcp/tool_contract_test.go	Adds the permissive-baseline narrowing regression and conditions simple optional-property compatibility on the v1 additional-properties policy.

🧪 Test Plan

• RED reproduced permissive additionalProperties optional addition error = <nil>.
• Focused comparator and verification tests pass after GREEN.
• go test ./internal/mcp -count=1 passes.
• MCP package coverage remains 88.8%.
• Fixture SHA-256 remains unchanged.

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🤖 Automated Checks

Fork-first note: the upstream issue is approved, but the fork-local governance job may not resolve an upstream issue through context.repo. The final upstream PR will use the canonical issue-closing reference.

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✅ Contributor Checklist

• The approved upstream issue is linked above.
• Exactly one PR type and matching label are present.
• Commit follows conventional-commit format.
• No Co-Authored-By trailer is present.
• The correction is one file and 15 changed lines.
• No fixture or production behavior changed.

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💬 Notes for Reviewers

This exceptional fourth fork PR is a bounded post-stack correction required by native high-risk RDD. The finding is semantic: naming and typing a previously arbitrary key narrows accepted inputs even when the new property is optional.