Fix streaming tool call nil arguments

[afurm]

What this does

Fixes a streaming crash when tool-call deltas omit function.arguments by normalizing nil/empty fragments during accumulation, and adds a regression spec to cover the case.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: #___

PRs for new features or enhancements without a prior approved issue will be closed.

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

[aka-nez]

Faced exactly the same issue with openrouter provider

[smerickson]

I had been running into issues using Haiku 4.5 at AWS Bedrock while streaming and using tool calls. The way it manifested was the model calling a tool with incorrect parameters. I always had a sneaking suspicion that it wasn't model hallucination because I had never experienced that when I was using the AWS SDK directly instead of RubyLLM. I just applied this patch to my system and my sreaming issues with Haiku seen to have been resolved. this would be great to get merged into core.

[afurm]

Thanks for confirming and for testing this.