fix(typescript-fetch): support non-default file names

[celluj34]

files generated with non-default (e.g. "fileNaming": "kebab-case") have their import path set with the default file name. this PR fixes the non-barrel imports by using the parsed file name instead of simply the class name.

(fixes #23840)

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Summary by cubic

Fixes broken model imports in the typescript-fetch generator when fileNaming is non-default (e.g., kebab-case). API files now import models using the actual generated file name instead of the class name.

• Bug Fixes
  • Generate and use classFileName (via convertUsingFileNamingConvention) for model import paths in apis.mustache and sagas.mustache to support non-default fileNaming.
  • Added a test in TypeScriptFetchClientCodegenTest to assert kebab-case imports (e.g., ../models/pet not ../models/Pet) and set WITHOUT_RUNTIME_CHECKS to false to exercise imports.
  • Added bin/configs/typescript-fetch-kebab-case.yaml and regenerated samples under samples/client/petstore/typescript-fetch/builds/kebab-case for coverage.

^{Written for commit 80e5496. Summary will update on new commits.}