feat(mcp): zod argument validation and prometheus metrics (#411, #425)

[sparkysolutions10-pixel]

Resolves #411
Resolves #425

Summary of Changes

This PR addresses two critical MCP infrastructure issues in Divinity-Labs/Lens:

1. Zod Argument Validation (Tool arguments are coerced with String() and never validated #411):

   • Validates tool arguments before execution using single-source-of-truth Zod schemas (BazaarSearchSchema and PaidCallSchema).
   • Prevents String(undefined) coercion to "undefined/discovery/resources". Missing or wrongly-typed arguments return a structured ERR_BAD_REQUEST naming the failing argument path (e.g. Missing or invalid argument "url": url is required).
   • Restricts paid_call method argument to documented set (GET, POST, PUT, DELETE, PATCH, HEAD), defaulting safely to 'GET'.
   • Derives advertised inputSchema directly from the Zod schemas using zodToInputSchema so the advertised schema and server runtime validation rules cannot drift.

2. Prometheus MCP Metrics (The MCP server has no metrics #425):

   • Adds mcp_tool_calls_total counter tracking tool calls by tool name and outcome (success | error).
   • Adds mcp_tool_call_duration_seconds histogram measuring execution duration per tool name.
   • Adds mcp_tool_errors_total counter tracking errors by tool name and error code (MCP_ERROR_CODES).
   • All labels are strictly closed and bounded (tool: 'bazaar_search' | 'paid_call' | 'unknown').
   • Metrics are registered on the existing prom-client registry and exported via /metrics (getMetrics()).

3. Vitest Test Suite:

   • Added unit tests in tests/mcp.test.ts covering missing/invalid arguments, out-of-range methods, method defaults, schema derivation parity, counter increments, duration observations, and /metrics registration.
   • All 14 tests passing in 54ms.
   • Changeset added under .changeset/mcp-zod-validation-and-metrics.md.

[Miracle656]

Changes requested: one issue per PR, and only issues assigned to you

This is a process requirement, applied to all 23 of your open PRs in this repo. It is not a judgement about the code, which I have not been able to review — and that is the first half of the problem.

The bundling

Across your open PRs, 150 distinct issues are claimed. This one claims several. #483 claims 22, #481 claims 20, #477 claims 20, #474 claims 20.

A PR that closes twenty issues cannot be reviewed. Each of those issues has its own acceptance criteria — typically five — so twenty issues is a hundred separate things to verify in one diff, and if any one of them is wrong the whole PR is blocked while it is fixed. Every batch I published says the issues are deliberately independent so they can be worked in parallel, by different people, without colliding. Bundling them removes exactly that property.

The assignments

135 of the 150 issues claimed are assigned to other contributors — 67 different people.

This PR is an example: it names issues currently assigned to other contributors who are working on them now. Several have open PRs of their own. If this merged as it stands, those people would have done the work and received nothing for it, and I would have no way to tell the two efforts apart.

I am not assuming anything about why. Assignments change, Drips and GitHub can disagree, and a wide-ranging branch picks up issue numbers easily. But the effect on the people holding those assignments is the same regardless of intent, which is why I have to act on it.

What I need

• One issue per PR. The issue number in the title, its acceptance criteria met, and nothing else in the diff.
• Only issues assigned to you, or unassigned ones you have claimed through Drips. If an issue is assigned to someone else, it is theirs.
• Re-open the work that meets both conditions as individual PRs. I will review them in the order they arrive and at the same pace as everyone else's.

Nothing is being closed. These stay open and are yours to re-cut, withdraw or split as you choose.

Two things worth knowing before you re-cut

Some of this has already landed. #488 (SSRF guard for bazaar_search) and #487 (the paid_call host allow-list) cover the same ground as your #449; #486 covers the MCP README. Check current main before rebuilding something that is already there.

main has moved a long way today — #306, #309, #252, #256, #355, #381, #390, #402, #476, #488 and more. Cut from current main, not from where these branches started.

If you believe an issue named here is genuinely yours, say which and I will check the assignment myself — I would rather be corrected than have someone lose work.