feat: allow RegExp validation patterns []

[Jared Jolton (jjolton-contentful)]

Summary

• Allow regexp.pattern and prohibitRegexp.pattern validations to accept JavaScript RegExp values.
• Serialize RegExp patterns to CMA-safe { pattern, flags } payloads before content type requests are created.
• Document and test flag precedence: explicit flags values override flags embedded in the RegExp.
• Update public TypeScript declarations and add regression coverage for validation + payload generation.

Description

This keeps the existing string-based validation shape intact while allowing migration scripts to pass a RegExp instance as the pattern value. The generated payload stores RegExp.source as pattern, includes RegExp.flags when no explicit flags value is provided, and lets explicit flags values win when both are present.

Generated with Codex

Motivation and Context

Fixes #1178.

The previous string-only API requires awkward double escaping in migration scripts. Accepting RegExp values lets users write the same expression they would normally use in JavaScript while preserving the CMA payload contract.

PR Checklist

• I have read the CONTRIBUTING.md file
• All commits follow conventional commits
• Documentation is updated (if necessary)
• PR doesn't contain any sensitive information
• There are no breaking changes

Test plan

• npm run build
• npm run test:unit
• npm run lint
• npm run test:typescript-declaration

[Tyler Pina (tylerpina)]

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