Add regex pattern validation

[cubahno]

Enforce OpenAPI pattern constraints in generated validation

Generated Validate() methods now respect the OpenAPI pattern (regex) keyword. Until now pattern was parsed but never enforced, so values that violated a schema's regex still passed validation.

What it does

• String fields, array items, and map values that declare a pattern are checked against the regex inside Validate().
• Empty values pass the pattern check (use required to reject empties), consistent with how the other constraints behave.
• Failures report the field and the expected pattern.

Notes

• validation.simple: true keeps using validate.Struct() and does not enforce pattern, in line with its other documented limitations.
• Patterns that Go's regex engine can't compile (e.g. lookahead/backreferences) are skipped with a warning instead of breaking generation.