fix: isolate embedding cache by model and prefix

[Daniel Peng (original4422)]

Summary

• key the class-level embedding cache by model, prefix, and normalized input
• share the same cache-key logic between sync and async embedding paths
• add deterministic sync and async regressions for configuration isolation and same-configuration cache hits

Root cause

EmbeddingSimilarity used only the normalized input as its class-level cache key even though the embedding request also depends on model and prefix. Instances with different configurations could therefore reuse vectors produced by another model or prefixed input.

Validation

• pytest py/autoevals/test_embeddings.py -k 'cache_isolated' -q (2 passed)
• pytest py/autoevals/test_values.py py/autoevals/test_json.py -q (8 passed)
• pre-commit run --all-files
• python -m build
• python -m twine check dist/*

The full local suite completed with 72 passed and 17 integration failures because OpenAI/Braintrust API credentials are unavailable locally; those failures stop at client initialization and do not exercise this cache change.

Fixes #211

[Copilot]

Pull request overview

This PR fixes a correctness issue in the Python EmbeddingSimilarity scorer where its class-level embedding cache could be inadvertently shared across different configurations, leading to cross-model / cross-prefix cache poisoning. It aligns with the library’s goal of providing reliable, deterministic evaluators by ensuring cached embedding results are only reused when the embedding request configuration matches.

Changes:

• Introduces a shared cache-key function for EmbeddingSimilarity and keys the cache by (model, prefix, normalized_input).
• Applies the same cache-keying behavior to both sync and async embedding paths.
• Adds regression tests to confirm cache isolation across models/prefixes and cache hits within the same configuration.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
py/autoevals/string.py	Fixes EmbeddingSimilarity cache keying to prevent cross-configuration reuse and shares key logic between sync/async paths.
py/autoevals/test_embeddings.py	Adds deterministic regression tests (sync + async) and a per-test cache reset fixture to validate isolation and same-config cache hits.

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