chat: fix magistral template detection and parser format
- Move magistral detection before ministral_3 in template matching
(both use [TOOL_CALLS][ARGS] patterns)
- Add "Unsloth" check to distinguish magistral templates
- Rewrite PEG parser to match actual template format:
[TOOL_CALLS]name[ARGS]{...} (not JSON array with ids)
- Update test to reflect format doesn't include tool call ids
Enables: magistral:legacy, magistral:experimental
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Co-Authored-By: Claude <noreply@anthropic.com>
