Rewrite workbench e2e tests for tabbed UI

- Add helpers: goToCreateTab, goToAgentsTab, createAgent, createAgentViaAPI, deleteAgentViaAPI, mockEmptyRuns
- Update 'creates and deletes' to use Create Agent tab and agent cards
- Update 'runs an agent' to verify output in RunsSidePanel
- Update 'requires input' to use card inline input field + Go button
- Update 'suggest schema' to navigate to Create tab first
- Update 'failure handling' to check error in run detail panel
- Refactor SchemaRenderer tests to use setupSchemaTest helper (API-created agents, run output in side panel)
- Keep Agent Chat UI and Show in Menu tests unchanged

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: abossard