Expose combineAbortSignals as public utility

[marcstraube]

Context

combineAbortSignals is used internally in RequestInterceptor but not exported for consumers who need to combine abort signals in custom workflows.

Goal

Expose as a public utility in the request module.

Implementation

1. Export combineAbortSignals from src/request/index.ts
2. Add TSDoc documentation
3. Add dedicated tests (currently only tested indirectly)

Files

• src/request/index.ts
• tests/request/combineAbortSignals.test.ts (new)

Source: AUDIT4 (-1 Feature-Completeness)