feat(decorators): accept RegExp in ApiProperty({ pattern }) while keeping schema pattern string

[Kimxxunu]

Allow passing a RegExp to ApiProperty({ pattern }). The decorator normalizes the RegExp to an
OpenAPI-compatible string by stripping slashes and flags (e.g., /^\w+$/gi → ^\\w+$). This keeps
the generated schema compliant with OpenAPI/JSON Schema where pattern must be a string.

• Input DX: pattern?: string | RegExp (options only)
• Output schema: pattern is always a string
• No breaking changes
• Added unit tests (5 cases) for normalization and immutability

Closes #3374

PR Checklist

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (not required for this change)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

ApiProperty({ pattern }) only accepts a string. When developers want to reuse an app-wide RegExp,
they must convert it to a string and manually strip the leading/trailing slashes and ignore flags,
which is repetitive and error-prone.

Issue Number: #3374

What is the new behavior?

ApiProperty({ pattern }) now accepts string | RegExp. When a RegExp is provided, the decorator
normalizes it to a plain string (no slashes/flags) before storing metadata, keeping the generated
OpenAPI schema valid.

Does this PR introduce a breaking change?

• Yes
• No

Other information

• Implementation localized to the decorator options processing:

  • lib/decorators/api-property.decorator.ts: accept pattern?: string | RegExp at options level,
    normalize via RegExp#toString() and remove slashes/flags before emitting metadata.
  • Does not change OpenAPI types: schema pattern remains a string.
  • Does not mutate the caller’s options object.

• Tests:

  • test/decorators/api-property.decorator.spec.ts
    • converts RegExp → string (removing slashes/flags)
    • keeps string pattern as-is
    • supports new RegExp(...)
    • preserves escaped slashes inside the body
    • does not mutate the original options object