[codex] fix MCP output schema contracts

[RhysSullivan]

Summary

Fixes the MCP output schema/display mismatch and keeps regression coverage around the full contract.

For MCP tools, upstream outputSchema describes the MCP result's structuredContent. Executor invocation data, however, is the full successful MCP CallToolResult object under ToolResult.ok.data: content, optional structuredContent, optional _meta, and optional isError. The MCP plugin now registers tool output schemas as that CallToolResult shape, nesting the upstream MCP schema under structuredContent.

The CallToolResult wrapper is derived from @modelcontextprotocol/sdk's CallToolResultSchema rather than hand-written. Executor only substitutes structuredContent with the per-tool upstream output schema and narrows successful results to isError: false, so content block details and future MCP schema changes track the SDK. Top-level extra properties remain permitted because MCP's Result type is intentionally extensible.

The PR also adds reusable output contract coverage through typeCheckOutputTypeScript, which compiles a described tool output TypeScript type against an actual runtime output value. MCP tests use it against real invocation data, and the existing core tool-invoker tests now share the helper too.

Root Cause

MCP discovery copied the upstream MCP outputSchema directly into Executor's registered tool schema. That made the schema panel and generated output TypeScript describe data as the upstream structured payload, while the actual invocation returned data as the full MCP call result envelope.

Validation

• bun run --cwd packages/core/sdk typecheck passes with existing Effect language-service warnings
• bun run --cwd packages/core/execution typecheck passes with existing src/promise.ts warning
• bun run --cwd packages/plugins/mcp typecheck passes with existing Effect language-service warnings
• ./node_modules/.bin/oxlint -c .oxlintrc.jsonc packages/plugins/mcp/src/sdk/plugin.ts packages/plugins/mcp/src/sdk/elicitation.test.ts packages/plugins/mcp/package.json bun.lock --deny-warnings
• bun run --cwd packages/core/sdk test -- src/testing/tool-output-contract.test.ts
• bun run --cwd packages/plugins/mcp test -- src/sdk/elicitation.test.ts -t "outputSchema|CallToolResult|refreshSource|structured MCP result"
• bun run --cwd packages/plugins/mcp test -- src/sdk/elicitation.test.ts

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commit: 5e7f807