Fix wrong import statements in dart-dio when using import-mappings

[bw-flagship]

Fixes #23558

@jaumard @josh-burton @amondnet @sbu-WBT @kuhnroyal @agilob @ahmednfwela

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---

Summary by cubic

Fixes wrong imports in dart-dio when using --type-mappings with --import-mappings. Generated code now imports/exports the mapped package path and skips generating duplicate model files.

• Bug Fixes
  • Use vendorExtensions.x-import-path for model imports/exports in the barrel file and serializers (built_value and json_serializable).
  • Detect models that are both type-mapped and import-mapped and do not generate their model files.
  • Add tests and a sample spec to verify custom paths are used in models, serializers, and the barrel file.

^{Written for commit 56a2062. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

[wing328]

cc @OpenAPITools/generator-core-team