Fix/issue 186 dispose cancel

[apravint]

Description

Fixes #186 - Prevents crash when users navigate away while LLM is streaming a response.

When a user navigates away from LlmChatView while the LLM is still streaming a response, the widget crashes with Null check operator used on a null value. This happens because the dispose() method doesn't cancel pending responses, allowing the onUpdate callback to call setState() on a disposed widget.

Changes

• Added _pendingPromptResponse?.cancel() in dispose()
• Added _pendingSttResponse?.cancel() in dispose()
• Moved super.dispose() to the end to ensure proper cleanup order

Impact

This fix resolves crashes affecting 588 occurrences across 124 production users in production apps using flutter_ai_toolkit 1.0.0.

Pre-launch Checklist

• I read the Contributor Guide and followed the process
• I read the Tree Hygiene wiki page
• I read the Flutter Style Guide recently
• I signed the CLA
• I listed at least one issue that this PR fixes (LlmChatView dispose() does not cancel pending streaming response, causing setState on disposed widget #186)
• I updated/added relevant documentation
• This change is straightforward and doesn't require new tests
• All existing tests pass

[apravint]

The fix adds the missing cancel() calls for both _pendingPromptResponse and _pendingSttResponse in the dispose() method to prevent setState on disposed widget.

The implementation:

1. Cancels pending LLM response stream
2. Cancels pending STT response stream
3. Moves super.dispose() to the end for proper cleanup order

This directly addresses the 588 occurrences affecting 124 users you reported.