Gaotax2006 [ Fix ] Fix #4424: Reject YAML config files with explicit error message

[Gaotax2006]

Fixes #4424

Detect .yaml/.yml files in Config.load() and raise ValueError with a clear message instead of letting json.load fail with a generic parse error.

Files changed

• src/common/config.py

Acceptance checklist

• Fix implemented as described
• Follows existing codebase conventions
• No AI training leakage markers
• No CONTRIBUTORS.json modifications
• No build artifacts committed

[Gaotax2006]

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