feat(ui): standardize searchable agent pickers

[sriaradhyula]

Description

Standardize compact single-agent selection on the shared searchable AgentPicker and ensure paginated Dynamic Agent catalogs are loaded completely.

Root cause

The Webex 1:1 and Conversations screens built picker options from only the first Dynamic Agents API page. Other screens used native or one-off selectors, so search and required-selection behavior were inconsistent.

What changed

• Add a shared paginated Dynamic Agents loader with page deduplication and API error handling.
• Use it for Webex 1:1 routing, Conversations filtering, and the Slack VictorOps setting.
• Replace the Conversations and Access Explorer native agent selects with the shared searchable picker.
• Add required-selection support to AgentPicker.
• Use the complete available-agent endpoint in Workflow Editor and new-agent subagent selection.
• Add unit and mocked Playwright coverage for search, required selections, and agents returned on page 2.

Type of Change

• Bugfix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other

Validation

• npm test -- --runInBand src/lib/__tests__/dynamic-agent-list.test.ts src/components/ui/__tests__/agent-picker.test.tsx src/components/admin/rebac/__tests__/WebexSpaceRebacPanel.test.tsx src/components/admin/security/__tests__/AccessExplorerTab.test.tsx — 27 passed
• RUN_RBAC_REGRESSION=1 npx playwright test e2e/rbac/webex-configure-spaces.spec.ts e2e/rbac/dynamic-agents-conversations-superadmin.spec.ts e2e/rbac/access-explorer.spec.ts --config=playwright.rbac.config.ts — 17 passed
• npm run lint
• npm run build

Checklist

• I have read the contributing guidelines
• I have verified this change is not present in other open pull requests
• All code style checks pass
• New code contribution is covered by automated tests
• All relevant tests pass