[typescript] Align multipart file array handling

[hwisu]

Summary

• Fix multipart file arrays for typescript-fetch, typescript-axios, and typescript-inversify.
• Treat form params as file arrays when parameter.items is binary.
• Move the shared check to AbstractTypeScriptClientCodegen.
• Add regression tests for the generated output.
• Document the shared binary form-array helper.

Root Cause

For type: array with items: { type: string, format: binary }, the file metadata is on the item schema, not the array schema. These TypeScript generators only checked parameter.dataFormat == "binary", so they could serialize file arrays as csv form values instead of repeated multipart fields.

Cross-Generator Consistency

The same OpenAPI shape is already covered by other generators:

• modules/openapi-generator/src/test/resources/3_0/form-multipart-binary-array.yaml defines files as an array of binary items.
• Spring generates List<MultipartFile> files with @RequestPart("files").
• JAX-RS/Jersey generates List<FormDataBodyPart> files.
• Java clients add repeated multipart entries with formParams.addAll("files", ...).
• JavaScript clients pass multipart binary arrays through instead of csv-joining them.
• Go tests cover array-of-binary operation parameters.

This change applies the same item-level binary check to the affected TypeScript clients.

Regression Safety

• typescript-axios and typescript-inversify use isCollectionFormatMulti for repeated field serialization.
• typescript-fetch also needs isFile so multipart requests use FormData.
• Regenerating the existing typescript-axios echo sample produced no diff.
• Regenerating the existing typescript-inversify petstore sample produced no diff.
• Added a focused typescript-fetch sample for this case.

Validation

• ./mvnw -pl modules/openapi-generator -am -Dtest=TypeScriptFetchClientCodegenTest,TypeScriptAxiosClientCodegenTest,TypeScriptInversifyClientCodegenTest -Dsurefire.failIfNoSpecifiedTests=false test
• ./bin/generate-samples.sh bin/configs/typescript-fetch-multipart-file-array.yaml
• ./bin/generate-samples.sh bin/configs/typescript-axios-echo-api.yaml
• ./bin/generate-samples.sh bin/configs/typescript-inversify.yaml
• git diff --check upstream/master...HEAD
• Earlier validation on this branch:
  • ./mvnw -pl modules/openapi-generator-cli -am -DskipTests package
  • ./mvnw clean package
  • ./bin/generate-samples.sh ./bin/configs/*.yaml
  • ./bin/utils/export_docs_generators.sh

Fixes #22597

PR checklist

• Read the contribution guidelines.
• Added generator-level regression coverage for the affected TypeScript clients.
• Rebased onto the fork default branch and kept commits scoped.
• This targets TypeScript clients and mentions recent TypeScript technical committee members for review.

Technical committee

This PR targets TypeScript clients (typescript-fetch, typescript-axios, and typescript-inversify).

cc @davidgamero @mkusaka @joscha @dennisameling

[cubic-dev-ai]

1 issue found across 18 files

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