Backport generator fixes from tum-apply

[az108]

Description

The standalone openapi-generator-angular21 repo has small issues which are fixed the version used in ls1intum/tum-apply, which has accumulated 11 bug fixes and improvements during integration. These need to be backported to keep the standalone generator usable by other projects.

Fixes to backport

1. Path template generation — Rewrote buildPathTemplate to use original OpenAPI paths instead of regex-parsing the parent's encodeParam format, which broke with openapi-generator 7.18.0+
2. GET methods in service classes — Removed the x-is-get filter so services include all HTTP methods, not just mutations
3. Model-to-model imports — Added ./ relative path prefix in model.mustache
4. API-to-model imports — Switched to tsImports with correct model filenames instead of imports which resolved to the API tag name
5. Array-of-enum types — Wrapped enum types in Array<> when the property is an array
6. Void return types — Added explicit <void> type parameter to HttpClient calls
7. Duplicate HttpHeaders import — Replaced per-operation conditional import with static import
8. Const enum objects — Generate runtime const objects alongside string union types for enum access
9. Blob response handling — Added responseType: 'blob' and observe: 'response' for file download endpoints
10. Array query params — Use URLSearchParams.append() instead of join(',') for correct wire format
11. Inline model filenames — Convert underscores to hyphens in toKebabCase() for consistent filenames
12. Model output folder — Use model/ instead of models/ for output directory

Acceptance Criteria

• All 4 source files match the tum-apply versions
• Generator compiles and produces correct output against a sample OpenAPI spec