[Nim] Fix nested map support

[winrid]

Fixes #22640
Fixes #22641

• Nested map support. (Table[string, Table[string, string]])
• Negative enum support (where abs value of ordinal clashed with other value).

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Summary by cubic

Adds nested map support and fixes enum handling for negative numeric values in the Nim client codegen to generate correct types and valid enum names. Fixes #22640 and #22641.

• Bug Fixes
  • Resolve $ref in getTypeDeclaration to detect nested maps/arrays and emit correct Table[...] types.
  • Detect numeric enums from integer/number schemas and prefix negative values with "Neg" to avoid identifier collisions.
  • Update petstore fixtures and generated Nim models (DigestEmailFrequency, PetStatistics) to cover these cases.

^{Written for commit c81ac9e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 6 files

[winrid]

@hokamoto would you be so kind as to review?