Add tests for rerankerService.rerank()

[felladrin]

Split off from #2130.

server/rerankerService.ts is only partially tested: server/rerankerService.test.ts covers sanitizeUnicodeSurrogates, but the core rerank() function has no tests.

Task: Add tests for rerank(), mocking only at the boundary (the model call) and asserting the real ranking behavior.

Pattern to follow: server/webSearchService.test.ts — mock at the boundary, assert real behavior. server/rankSearchResults.test.ts (which mocks rerank) is also a useful reference for the shapes involved.

Good first issue: narrow scope, a single function, and existing tests in the same file to copy from.

[felladrin]

Closing this: the tests landed in rerankerService.test.ts (line 303), covering empty/null/undefined input, model-not-loaded, and basic scoring. The core is covered.