feat(agent): include accumulated text in OnTextEnd callback

[gdemoni]

Summary

• pass the fully accumulated text to OnTextEndFunc
• update the callback signature to func(id, text string) error
• add regression coverage for concatenated deltas and interleaved text IDs

This follows the approach proposed in #240, with additional regression coverage.

Compatibility

This changes the public OnTextEndFunc signature. Existing callbacks need to accept the new text parameter.

Testing

• go test ./... -count=1 -timeout=30m
• golangci-lint run

Closes #239

• I have read the CONTRIBUTING.md file（https://github.com/charmbracelet/.github/blob/main/CONTRIBUTING.md）

[gdemoni]

Hi maintainers! @andreynering
It looks like the workflows for this PR are awaiting approval. When someone please approve them when convenient so the required checks can run? Thank you!

[gdemoni]

@andreynering
Please review my changes. If they don't introduce any new issues and effectively improve the project's bugs, please merge them. Thank you.