Ensure consistent float type for cached embedding return values

[kazuya-awano]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

This PR ensures that the return values of embeddings retrieved from the cache are consistently in float type. This change prevents potential issues caused by type mismatches (float vs float64) during serialization, especially when using Chroma as the vector database.

Fixes #7213

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

• Test
  • Set up Chroma as the vector database
  • Run a Retrieval Testing
    • Results are retrieved successfully.
  • Run the same search again
    • Before Fix: Prior to this fix, running the search twice would result in an error: Type is not JSON serializable: numpy.float64.
    • After Fix: After applying this fix, the same search should succeed without errors, and the results should be retrieved consistently.