fix: repair pool credential handling and connection selection

[ctkm-aelf]

Summary

• Fix service-pool inventory and health requests failing with Bad request: API key is failed when one connection has an unusable stored credential. Report that connection as unavailable and let priority pools dispatch to a healthy backup, while preserving direct-proxy HTTP 400 responses and genuine database errors.
• Replace the connection list with a searchable multi-select dropdown that stays open while selecting, supports deselection and keyboard navigation, and keeps member configuration in the form. Search connection names and slugs before pagination. Require a selected connection for new dashboard pools while preserving CLI/API empty drafts. Keep the picker above selected cards to prevent ResizeObserver feedback as selections change.
• Add regressions for credential states, agent overrides, scope and transport behavior, search pagination, error reporting, and dropdown interactions. Update the API and pool documentation.

Follow-up to #1731. Related to #1680.

Test Plan

• 113 backend service-pool tests, 131 proxy-service tests, 21 error-contract tests, and 14 proxy-telemetry tests passed against local MongoDB 8 where required. The final ten inspection tests passed after adding name search.
• Frontend: 4,113 tests in 411 files, including 31 focused pool tests; lint and production build passed.
• Final independent real server/CLI/browser smoke: 11 checks passed for candidate inventory, operation checks, name search before pagination, saved health, one-attempt healthy fallback, direct error compatibility, and empty drafts.
• Real browser create/edit/delete passed against the local backend, including displayed-name search, multi-selection, deselection, unavailable connections, repeated Escape/focus restoration, desktop/mobile layout, and zero window, page, or console errors in the production build.
• Permanent browser regression: the previous placement failed all three runs with the exact ResizeObserver error; the corrected layout passed all five runs. The final 31 focused frontend tests, TypeScript, lint, and production build passed.
• Workspace Clippy, Rust formatting, and the backend boundary check passed.
• Final-head CI (8d6a5a50): CI Pipeline, all CodeQL jobs, frontend and backend coverage passed. Backend CI ran 7,140 passing tests (5 skipped by the suite). Squash-merged into main as 89257aa5.

Checklist

• Code follows the project's architecture rules.
• No hardcoded secrets or credentials.
• Error messages do not leak internal details.
• Documentation updated.

[github-actions]

📊 Code coverage

Component	Lines	Threshold	Status	Δ vs base
Backend (nyxid)	87.55%	73%	✅	🔺 +0.02
Frontend (vitest)	71.89%	15%	✅	🔺 +0.02

Gate: line coverage must stay at or above the threshold. Ratchet plan (W21): Backend → 55%, CLI → 50%, Frontend → 30% by quarter end.