fix: handle whitespace between <tool_call> tag and JSON in Hermes handler

[ipmsteven]

Summary

• Fix Hermes handler parse() failing to extract tool calls when whitespace (space/newline) exists between <tool_call> and the JSON object
• This format is produced by Qwen models per their Jinja template: <tool_call>\n{"name":...}\n</tool_call>

Root Cause

_openRegex group(3) captures leading whitespace as part of the match (e.g., {"name"). When computing jsonStart, the offset pointed to the whitespace character instead of {, causing extractLeadingJsonValue() to return null and the entire tool call to fall back as plain content.

Fix

Skip leading whitespace after computing jsonStart to find the actual { before calling _extractJsonObjectRange(). This is consistent with how Granite, Mistral, and Magistral handlers already handle whitespace before their extractLeadingJsonValue() calls.

Test plan

• 3 new test cases: space, newline, and mixed whitespace between tag and JSON
• All 1027 existing tests pass
• Verified end-to-end with Qwen3.5-2B on iOS simulator

[leehack]

Thanks for the contribution, @ipmsteven!

It seems there's a very minor issue. Would you mind fixing the formatting? I can merge it as soon as that's addressed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.90%. Comparing base (ff0be57) to head (95190f9).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   77.23%   76.90%   -0.34%     
==========================================
  Files          67       67              
  Lines        8338     8368      +30     
==========================================
- Hits         6440     6435       -5     
- Misses       1898     1933      +35     

Flag	Coverage Δ
unittests	76.90% <100.00%> (-0.34%)	⬇️

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[ipmsteven]

Thanks for the contribution, @ipmsteven!

It seems there's a very minor issue. Would you mind fixing the formatting? I can merge it as soon as that's addressed.

formatting issue should be fixed now 👍