test: add testcases and coverage

[hustcc]

• test: add testcases and coverage 96%+.
• add HF_ENDPOINT tip

[gemini-code-assist]

Code Review

This pull request significantly expands test coverage across multiple modules (including the embedder, expander, loaders, reranker, storage, and utilities) and reorganizes utility functions, such as moving containsCJK to the tokenizer module. The reviewer feedback highlights several important improvements: cleaning up temporary directories in test/context.test.ts to prevent workspace clutter, reordering setup steps in test/storage/store.test.ts to avoid test instability, using cross-platform paths instead of hardcoded absolute paths in test/utils/sample.test.ts, and correcting the documented type of queryExpansion in the README to match its actual implementation.

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[hustcc]

@copilot ci 报错了 https://github.com/antvis/context/actions/runs/28921204212/job/85800697202?pr=4

[Copilot]

@copilot ci 报错了 https://github.com/antvis/context/actions/runs/28921204212/job/85800697202?pr=4

The failures were caused by two issues:

1. Timeout too short: The 30s testTimeout/hookTimeout wasn't enough for the HuggingFace model (~86MB) to download on a fresh CI runner. The main Context beforeAll timed out → 14 tests skipped. Then Context with reranking / Context with query expansion tests independently tried to load the model and also timed out. Increased both timeouts to 120s in vitest.config.ts.

2. Concurrent model downloads: When the beforeAll timed out mid-download, subsequent tests found Embedder.pipeline still null and each started its own download. Fixed Embedder.getPipeline() to cache the in-flight Promise (via static pipelinePromise), so all concurrent callers share a single download. On failure, both fields are reset to null to allow retries.

Commit: fix: increase test timeouts and prevent concurrent model downloads in CI