fix(agentflow): fix variable resolution, credential sync, and add E2E example (#6245)

* fix(agentflow): fix variable resolution, credential sync, and add E2E example

- Fix variable picker emitting {{nodeId.data.instance}} instead of {{nodeId}}
- Fix customMention serializer using display label instead of node ID
- Fix Start node missing outputData.content for chatInput type
- Sync credential selection to nodeData.credential for server-side execution
- Add E2E example with save/delete/test-run FABs via canvasActions
- Simplify BasicExample to minimal canvas-only usage
- Fix act() warnings in TooltipWithParser and useAsyncOptions tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(agentflow): add user-facing error feedback for save failures in E2E example

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(agentflow): fix variable chip display and revert Start node content field

- VariableInput: show {{nodeId}} in mention chip instead of display label,
  consistent with the value serialized to flow data
- useAvailableVariables: exclude startAgentflow from upstream node outputs;
  users reference the question via {{question}} (global variable), matching
  agentflow v2 behavior — no Start node change or server change needed
- Start.ts: revert outputData.content = input; Start node is no longer
  exposed as a node output variable in the picker

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>