fix: append error tool message when tool call is cancelled

[fuleinist]

Summary

When a user cancels a tool call by pressing Esc twice, the assistant message with tool_calls remains in the conversation history but no tool response follows. This causes 400 Bad Request errors from model providers on the next message.

The fix uses the parent context (ctx) instead of the request context (genCtx) when updating messages during tool call tracking and tool result creation. This ensures that even if the request is cancelled mid-stream, the message updates and creations succeed.

Additionally, the error message was updated from 'Tool execution canceled by user' to 'Error: user cancelled assistant tool calling' to better indicate the cancellation occurred during the assistant tool calling phase.

Changes

• OnToolInputStart: Use parent ctx instead of genCtx for messages.Update
• OnToolCall: Use parent ctx instead of genCtx for messages.Update
• OnToolResult: Use parent ctx instead of genCtx for messages.Create
• Updated error message to 'Error: user cancelled assistant tool calling'

Testing

• Existing tests pass (go build succeeds)
• Fix ensures tool call cancellation properly creates error tool message

Fixes #529
Fixes #1206

[charmcli]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[fuleinist]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[andreynering]

Looks good. Thank you @fuleinist!