[codex] Fix false positives for JsonRpcErrorCode comparisons

[ded-furby]

Summary

• scope JsonRpcErrorCode.X detection in lintErrorContractConformance to throw new McpError(...) sites instead of any enum reference in the handler source
• add a regression test covering err.code === JsonRpcErrorCode.NotFound followed by ctx.fail(...)

Root cause

The conformance heuristic treated every JsonRpcErrorCode.X token as a direct throw, so comparison logic inside catch/switch flows triggered error-contract-prefer-fail even when the handler routed correctly through ctx.fail(...).

Validation

• bunx vitest run --project unit tests/unit/linter/error-contract-rules.test.ts
• bunx vitest run --project unit tests/unit/linter/*.test.ts

Closes #191.

[cyanheads]

Thanks for the PR! We landed the same fix in v0.9.20 (#191) — scoped the scan to new McpError(JsonRpcErrorCode.X) construction (matching the prefer-error-factory rule, so it also covers the assign-then-throw case) plus a docs update. Closing since it is already shipped.