[BUG] typescript-angular: Generated service does not encode data correctly

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The Angular TypeScript generator produces code that incorrectly handles complex objects when the API expects form-encoded data (application/x-www-form-urlencoded or multipart/form-data). The generated code attempts to append complex objects directly to HttpParams, resulting in [object Object] being sent instead of properly serialized data.

OpenAPI Specification

paths:
  /api/BuchungsmaskeBelegRequest:
    post:
      operationId: apiBuchungsmaskeBelegRequestPost
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                buchung:
                  $ref: '#/components/schemas/BuchungslisteDto'
          application/json:
            schema:
              type: object
              properties:
                buchung:
                  $ref: '#/components/schemas/BuchungslisteDto'
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/BuchungsmaskeBelegResponse'

components:
  schemas:
    BuchungslisteDto:
      type: object
      properties:
        mandant:
          type: integer
          nullable: true
        buchungId:
          type: integer
          nullable: true
        buchungdatum:
          type: string
          nullable: true
        buchungen:
          type: array
          items:
            $ref: '#/components/schemas/SplitBuchungDto'
          nullable: true

Generated Code Issue

Current Generated Code

public apiBuchungsmaskeBelegRequestPost(buchung?: BuchungslisteDto, ...) {
    // ...
    const consumes: string[] = [
        'multipart/form-data',
        'application/json'
    ];
    
    const canConsumeForm = this.canConsumeForm(consumes);
    
    let localVarFormParams: { append(param: string, value: any): any; };
    let localVarUseForm = false; // ← HARDCODED TO FALSE
    let localVarConvertFormParamsToString = false;
    
    if (localVarUseForm) {
        localVarFormParams = new FormData();
    } else {
        localVarFormParams = new HttpParams({encoder: this.encoder});
    }
    
    if (buchung !== undefined) {
        // This appends the complex object directly, resulting in [object Object]
        localVarFormParams = localVarFormParams.append('buchung', <any>buchung);
    }
    
    // ...
    return this.httpClient.request<BuchungsmaskeBelegResponse>('post', url, {
        body: localVarFormParams, // Sends "buchung=[object Object]"
        // ...
    });
}

Problems

1. localVarUseForm is hardcoded to false: Despite calculating canConsumeForm, the variable localVarUseForm is always false, preventing the use of FormData for multipart uploads.

2. Complex objects are not serialized: When using HttpParams, complex objects are converted to [object Object] string instead of being properly serialized.

3. No Content-Type header is set: The generated code doesn't set an appropriate Content-Type header (should be application/x-www-form-urlencoded when using HttpParams or multipart/form-data when using FormData).

Expectations

When consumes is application/json the content-type should also be set correctly and data should be sent as json

openapi-generator version

7.14.0
with @openapitools/openapi-generator-cli 2.20.2

Generation Details

openapi-generator generate -i api.joson -g typescript-angular -o /src/app/gen/api

Steps to reproduce

1. Create an OpenAPI spec with a POST endpoint accepting both multipart/form-data and application/json
2. Define a request body with a complex object type
3. Generate Angular TypeScript code using openapi-generator-cli
4. Try to send a complex object using the generated method
5. Observe that the request body contains [object Object] instead of the actual data

Related issues/PRs

Suggest a fix

Reproduce the supplied OpenAPI specification with the typescript-angular generator using the command in the Generation Details, then inspect the generated service method's form and request-body handling. No source file or test is named; done means complex form data is not emitted as [object Object], multipart and urlencoded requests use appropriate handling, and application/json sends JSON with the correct content type.