fix: filter whitespace-only content for all providers

[nathannewyen]

Summary

• Trim whitespace from text content before checking if empty
• Filter whitespace-only messages and text parts for all providers
• Add tests for whitespace-only content handling

Fixes #8903

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found

PR #7086: "fix(provider): trim whitespace and filter empty content in transformations"
#7086

This PR appears to address the same issue - filtering and trimming whitespace content across provider transformations. The current PR #8954 is likely implementing a similar or identical fix. You should:

1. Check the state of PR fix(provider): trim whitespace and filter empty content in transformations #7086 (is it open, closed, or merged?)
2. If it's already merged, confirm whether PR fix: filter whitespace-only content for all providers #8954 is addressing the same code or a different aspect
3. If it's open, consider consolidating efforts or marking one as a duplicate

[rekram1-node]

pretty sure glm models have a weird behavior where they do empty text that actualy has to get sent back or something I remember there being some weird bug last time I tried this