feat!: Structured output

[gregnr]

Implements outputSchema / structuredContent for MCP tools (AI-847) which implicitely fixes execute_sql doubling backslashes in results (#311).

• Tool outputSchema is now returned as part of tools/list
• mcp-utils validates every tool's structuredContent against its outputSchema
• Return values from each tool's execute function feed directly into structuredContent
• Tools now have an optional formatResult hook that renders the returned structuredContent as text content (defaults to JSON.stringify)
• execute_sql returns rows as structuredContent and the prompt-injection fence now lives only in text content via formatResult. This removes the second JSON encode that was doubling backslashes in execute_sql corrupts backslash escaping in PL/pgSQL function bodies (round-trip failure) #311 (which confused LLMs)

The main downside to doing this is we lose the prompt-injection fence if the MCP client decides to pass the structured content directly to the LLM instead of the text-formatted version. I think this is a fair trade-off though - it matches the spirit behind structuredContent (actual structured content, not wrapped) and doesn't double JSON encode data. Client side prompt injection mitigations are a lot better than they used to be, so clients who want to use structuredContent should be willing to own this side of the security equation. Otherwise they can use the text version and continue to receive the wrapped version.

Closes AI-847
Closes #311

[coveralls]

Coverage Report for CI Build 28470473556

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Coverage: 96.248%

Details

• Patch coverage: 14 of 14 lines across 2 files are fully covered (100%).

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Coverage Stats

Relevant Lines:	2824
Covered Lines:	2739
Line Coverage:	96.99%
Relevant Branches:	348
Covered Branches:	314
Branch Coverage:	90.23%
Branches in Coverage %:	Yes
Coverage Strength:	43.13 hits per line

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