fix: getRegistryPrompt never validates its own declared-required prompt arguments

[JSONbored]

Goal

Make getRegistryPrompt validate the required arguments its own PROMPT_DEFINITIONS metadata declares, instead of silently interpolating missing values as placeholder text.

Why this matters

This is the one dispatch surface in the MCP package with its own required-field metadata that doesn't enforce it — every other tool handler validates required args and returns a proper error.

Current behavior

packages/mcp/src/registry-prompts-lib.js:

• PROMPT_DEFINITIONS (lines 11-72) marks use_case (asset.find), category/name (submission.prepare), draft (submission.review), and category/slug (install.asset) as required: true.
• getRegistryPrompt (lines 84-140) never checks presence of these — missing values are interpolated as literal placeholder text ("(not provided)", "(category)", "(slug)", "(draft not provided)") directly into the guidance sent back to the MCP client, instead of returning an error.
• Wired in via packages/mcp/src/server-lib.js:98 (getRegistryPrompt(request.params || {})) with no other validation layer in between.
• Every tool handler elsewhere in the package (getEntryDetail, getCompareEntries, etc.) validates required args and returns an invalid() envelope on failure — the prompt path is the outlier.

Desired behavior

getRegistryPrompt checks each prompt's declared required args before building the response, and returns a proper error/invalid envelope (consistent with how tool handlers elsewhere in the package report missing required args) instead of silently degrading into placeholder text.

Scope

• packages/mcp/src/registry-prompts-lib.js (getRegistryPrompt)
• focused tests

Out of scope

• PROMPT_DEFINITIONS's existing required-field declarations (already correct)
• Tool handlers that already validate correctly

Acceptance criteria

• PR includes Closes #<issue>.
• Calling any prompt with a missing required argument now returns an error/invalid response instead of a placeholder-filled guidance string.
• Calling with all required arguments present behaves exactly as before.

Quality evidence required in the PR

• API/MCP invariant note: list the exact error shape returned for a missing-required-arg call, and confirm it doesn't break existing valid-call behavior.

Validation

pnpm test:mcp
pnpm build
git diff --check

[linear-code]

JSO-7327