[Python] [Client] Add so that Python mapNumberTo supports Decimal

[Mattias-Sehlstedt]

Add so that the Python generator support so that the number type can be mapped to Decimal. Note that the is strictly for the Python generator, and not for the python-pydantic-v1 generator. This is intentional since they are based upon different AbstractCodegens that do not share their logic. If it is of interest to have all Python generators share this new behavior I would suggest creating a shared type resolver so that all Python generators could get the same functionality without code duplication.

The mapNumberTo function is retained as a scoped (argument-limited) function rather than a completely open one. This since the Python validation logic required outside knowledge that is currently not injectable by the client, so having an open function might currently be counterintuitive.

Fixes: #23887

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Summary by cubic

Adds support for mapping OpenAPI number types to Decimal in the python client via mapNumberTo, enabling high‑precision values (e.g., money) while preserving validation. Fixes #23887.

• New Features

  • mapNumberTo now accepts "Decimal" (supported: "float", "StrictFloat", "Union[StrictFloat, StrictInt]", "Decimal") and keeps numeric constraints.
  • Generates correct imports/types; clearer error enumerates supported values.
  • Docs and CLI help updated to list Decimal; tests use a data provider to cover all mappings.
  • Scope: python generator only; not python-pydantic-v1.

• Migration

  • Set mapNumberTo: "Decimal" in the generator config (Python generator only).

^{Written for commit d38ee4e. Summary will update on new commits.}