Fix: remove tool_choice: auto param for open ai

[imtyM]

What this does

Removes the tool_choice: 'auto' from Openai's render_payload which allows it to be overwritten with with_params.
Open defaults to auto when tools are present.

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

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

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

Related issues

Fixes #317

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.66%. Comparing base (b52caf2) to head (80b134b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #336      +/-   ##
==========================================
- Coverage   87.67%   87.66%   -0.01%     
==========================================
  Files          88       88              
  Lines        3473     3471       -2     
  Branches      724      723       -1     
==========================================
- Hits         3045     3043       -2     
  Misses        428      428              

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