Fallback to memory cache when disk is not available

[TomeHirata]

Resolve #8717

[Copilot]

Pull Request Overview

This PR implements a fallback mechanism for the DSPy cache to gracefully handle environments where disk cache cannot be initialized (e.g., AWS Lambda). When disk cache creation fails, the system now falls back to a memory-only cache instead of failing completely.

Key changes:

• Added a _get_dspy_cache() function that wraps cache initialization in a try-catch block
• Modified cache initialization to fallback to memory-only cache when disk cache fails
• Added test coverage for the fallback behavior in restricted environments

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dspy/clients/init.py	Implements cache fallback logic with new _get_dspy_cache() function
tests/clients/test_cache.py	Adds test case for cache fallback behavior in restricted environments

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