fix(tui): keep the /mcp panel border intact on multi-line server errors

[caigee-cmd]

Related Issue

Resolve #510

Problem

When a failed MCP server reports a multi-line error, the /mcp status panel's
border breaks. The panel (UsagePanelComponent.render) treats each line it is
given as exactly one boxed row, so an embedded newline — e.g. the
\nstderr: ... a failed stdio server appends — drops the trailing text to
column 0 and punches through the rounded border.

Repro (from #510): a stdio MCP server that fails to start returns something like
MCP error -32000: Connection closed\nstderr: usage: ...; running /mcp makes
that error row overflow the rounded box.

What changed

• Added formatErrorLine in mcp-status-panel.ts, which trims the error and
  folds every run of whitespace (including newlines) to a single space, keeping
  it on one row that the panel then truncates to the available width.
• Applied it to the rendered error: row.
• Added unit tests covering the multi-line fold and the single-line trim cases.

Note: the same newline-breaks-the-box hazard exists structurally in the shared
UsagePanelComponent renderer (it also affects the /goal reason row). This PR
intentionally scopes the fix to the reported /mcp symptom; happy to harden the
renderer for all panels in a follow-up if preferred.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset. (patch -> @moonshot-ai/kimi-code)
• Ran gen-docs skill, or this PR needs no doc update. (bug fix, no user-facing doc change)

Verification

pnpm run lint && pnpm run sherif
pnpm run test
pnpm run build && pnpm -C apps/kimi-code run smoke
pnpm --filter @moonshot-ai/kimi-code exec tsc --noEmit

[changeset-bot]

🦋 Changeset detected

Latest commit: f2c0dfe

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@f2c0dfe

npx https://pkg.pr.new/@moonshot-ai/kimi-code@f2c0dfe

commit: f2c0dfe