#24367 [rust-server] Fix client serialization of enum parameters in form request bodies

Fixes https://github.com/OpenAPITools/openapi-generator/issues/24366.

Issue restatement

For endpoints with request bodies of type form (e.g. application/x-www-form-urlencoded) and enum properties, the generated Rust client invoked format!("{:?}", param_foo) where param_foo is the model generated in models.rs for the enum property. {:?} is the debug formatter, which uses the derived impl Debug on the enum model, which resolves to the name of the enum variant (as per the rust docs) - however, we should instead be serializing this with the impl Display on the enum model, which produces the defined allowable values from the API definition. The old incorrect serialization did not obey the OpenAPI spec and caused any server policing the value of the enum to reject requests created by clients created using this tool.

Fix

Check if a parameter is enum-like using getIsEnumOrRef() when deciding how to format it via x-format-string, and instead format enums with {} (which will invoke their impl Display).

This is

• safe because all models impl Display
• correct, because all enums have allowedValues in their impl Display, populated from the openapi definition, and this is what should be serialized.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
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  These must match the expectations made by your contribution.
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  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
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Summary by cubic

Fix Rust server generator to serialize enum values in application/x-www-form-urlencoded bodies using Display, including enums inside arrays (required and optional), so clients send allowed OpenAPI values. Also standardizes example server logging to use {:?} for all params and regenerates samples.

• Bug Fixes
  • Enum-like form params now use {} via getIsEnumOrRef() checks on both params and array items, not {:?}.
  • Added test verifying required and optional array-of-enum form params use Display (no Debug) serialization.