test: add unit tests for rerankerService.rerank()

[felladrin]

Description

Fixes #2167.

rerank() had no unit test in the default suite. sanitizeUnicodeSurrogates was covered, and there's a rerankerService.integration.test.ts that runs rerank() against the real model, but that one downloads ~130MB and is excluded from npm run test. So the everyday suite exercised none of rerank()'s own logic: index mapping, batching, the empty/not-ready guards, and the sanitization step.

This adds fast, boundary-mocked tests for it.

One note on the boundary: the issue points at webSearchService.test.ts as the pattern, but that file mocks fetch. The reranker's boundary is the ONNX InferenceSession (plus the tokenizer), so these tests mock InferenceSession.create and spy on Tokenizer.prototype.encode instead. Same idea (mock at the boundary, assert real behavior), different seam.

What the tests check:

Test	What it verifies
empty / null / undefined documents	Returns [] early, without calling the model
service not ready	Throws Reranker service is not ready
score-to-index mapping	Each score lands on its document's index, in input order (scores are non-monotonic, so this also confirms rerank() does not sort; that lives in rankSearchResults)
batch splitting	25 docs run in 3 model calls (10 + 10 + 5), and the scores concatenate back in order across every batch boundary
exact multiple of BATCH_SIZE	20 docs run in exactly 2 calls, with no trailing empty batch
surrogate sanitization	A lone surrogate reaches the tokenizer as U+FFFD, never raw, proving sanitization runs on the real path and not just that the model was hit

The warm-up call startRerankerService() makes is cleared before each test, so the call-count assertions start from zero.

Type of Change

• Bug fix
• New feature
• Documentation
• Other (tests)

How to test

1. Run npm ci.
2. Run npx vitest run server/rerankerService.test.ts (or the whole suite with npm run test).
3. Run npm run lint.

Checklist

• npm run lint passes
• Tests pass (npm run test)

Security, performance, or breaking changes

None. Tests only, no production code touched.