feat(agentflow): enhance state management with StateKeyValueInput and variable support (#6119)

* feat(agentflow): enhance state management with StateKeyValueInput and variable support

- Added StateKeyValueInput component for managing key-value pairs in flow state updates.
- Integrated VariableInput for value fields to support variable syntax.
- Updated MessagesInput and ExpandTextDialog to utilize variable items for enhanced user experience.
- Introduced new types for state updates in core types.
- Enhanced Jest tests to cover new functionality and ensure reliability.

This update improves the flexibility and usability of state management within the agentflow package.

* fix(agentflow): address PR #6119 review feedback

- Add autoFocus prop to VariableInput, apply in ExpandTextDialog for
  consistent focus behavior with RichTextEditor fallback
- Extract duplicated VariableItem→SuggestionItem mapping into shared
  toSuggestionItems utility, used by MessagesInput and NodeInputHandler
- Remove redundant default export from StateKeyValueInput

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* clear selected key and update dropdown when a state key is removed

* cleanup unused type

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>