Fix PHP transformer text color class validity

[chubes4]

Summary

• Keep the reserved WordPress preset color slug text as a custom color style instead of serializing it to textColor: text, which avoids core generating duplicate has-text-color classes.
• Strip generated color support classes from source className when equivalent block color support attrs already own those classes.
• Add regression coverage for Coffee/CV-style paragraph color support class validity.

Verification

• php tests/unit/block-style-support-conversion.php
• composer test
• Transformer guard over Coffee, Artist, and CV fixtures confirmed duplicate_has_text_color=0 for all three.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Investigated SSI fixture-matrix Coffee residual evidence, identified the generic PHP transformer color-class validity issue, implemented the fix and regression tests, and ran local verification.