fix(settings): surface the server's validation detail on Add MCP Server failure

[AmirF194]

Summary

Follow-up to #6215. After merging, reviewer o3LL noted that static/js/settings.js's Add MCP Server failure branch still shows a generic Failed (${r.status}) instead of the server's actual validation message, and flagged it as fine for a separate PR. routes/mcp/mcp_routes.py's add_server() returns HTTPException(400, "...") for every validation failure (missing command, missing URL, malformed Args), and FastAPI puts that message in the response body's detail field. static/js/admin.js's own Add MCP Server handler already reads it (data.detail || \Failed (${res.status})``, admin.js:2391); settings.js's copy of the same form never picked it up. This fix mirrors that existing line exactly.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #6282

Type of Change

• Bug fix (non-breaking, fixes a confirmed issue)

Checklist

• I searched open issues and open PRs, this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above, no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up or uvicorn app:app) and verified the change works end-to-end. Type-checks and unit tests are not enough.
• I did not run the app/runtime validation and stated that gap in How to Test.

How to Test

1. node --check static/js/settings.js passes.
2. Extracted the exact uf-mcp-save click handler, the real source lines rather than a rewritten copy, from both origin/dev and this branch, and ran each in a Node vm sandbox with a mocked 400 response carrying {"detail": "command is required for stdio transport"}: the dev version renders Failed (400), this branch renders the server's actual message. The other three branches of that if/else (needs_auth, connected, plain Saved) are untouched by the diff.
3. Not run: the actual app end-to-end, so no screenshot below. This module has no existing test harness reaching initUnifiedIntegrations's MCP form (tests/helpers/test_settings_shell*.js cover the extracted settings/*.js shell modules, a different file), and the equivalent line in admin.js is likewise untested today.

Visual / UI changes, REQUIRED if you touched anything that renders

• Screenshot or short clip of the change in the running app, not attached, see point 3 above.
• Style match: no new CSS, colors, or component patterns; reuses the existing #uf-mcp-msg status line and the exact fallback already used at admin.js:2391 for the same failure.
• No new component patterns.
• I am not an LLM agent submitting a bulk PR. Per this note's own instruction for that case: the issue (settings.js Add MCP Server failure path shows generic status instead of the server's detail message #6282) was opened first, describing the problem, before this PR.

[github-actions]

⚠️ PR description is complete; validation evidence is still outstanding

Changed-file classification: UI-sensitive.

Author-reported runtime / visual state

• The author explicitly reports that app/runtime validation was not performed.
• The screenshot/clip checkbox is not checked for this UI-sensitive change.
• The Screenshots / clips section does not contain an actual attachment or link.

Checkboxes are author attestations. GitHub Actions results remain the execution evidence for CI; this check does not prove that a local command ran.

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