Fix schema JSON parsing for intermediate tool-call responses

[trouni]

Summary

Fixes #649

When both with_schema and with_tool are used together, the complete method unconditionally parses any string response.content as JSON whenever a schema is set — including intermediate responses that also contain tool calls.

On the next API call, the parsed Hash gets serialized as { type: "text", text: <Hash> }, which providers like Anthropic reject with:

text.text: Input should be a valid string

Root cause

In lib/ruby_llm/chat.rb, the JSON parsing condition:

if @schema && response.content.is_a?(String)

does not check whether the response is an intermediate tool-call response, so it eagerly parses content that should remain as a plain string.

Fix

Add a !response.tool_call? guard so JSON parsing is only applied to the final (non-tool-call) response:

if @schema && response.content.is_a?(String) && !response.tool_call?

Test

Added a regression test that mocks the provider to return an intermediate tool-call response with JSON-like text content, verifying that:

• The intermediate tool-call message keeps its content as a String
• The final non-tool-call response has its content parsed into a Hash

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.34%. Comparing base (363d3a7) to head (b7eac35).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #650      +/-   ##
==========================================
- Coverage   81.36%   81.34%   -0.02%     
==========================================
  Files         116      116              
  Lines        5425     5425              
  Branches     1426     1426              
==========================================
- Hits         4414     4413       -1     
- Misses       1011     1012       +1     

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