Fix embedding return format inconsistency for single-string arrays

[finbarr]

Summary

• Fixes inconsistent return format when passing an array with a single string to the embed method
• Ensures RubyLLM.embed([string]) returns [[vector]] instead of [vector]
• Maintains backward compatibility for single string inputs

Problem

As reported in #254, the embedding API had inconsistent behavior:

RubyLLM.embed(string) -> [vector] ✓
RubyLLM.embed([string, string]) -> [[vector], [vector]] ✓
RubyLLM.embed([string]) -> [vector] ✗ (should be [[vector]])

Solution

• Modified the base Provider class to pass the original text parameter to parse_embedding_response
• Updated OpenAI and Gemini providers to check if input was an array before unwrapping single embeddings
• Added comprehensive test coverage for this edge case

Test plan

• Added new test case specifically for single-string arrays
• All existing tests pass
• Verified fix works for both OpenAI and Gemini providers

Fixes #254

🤖 Generated with Claude Code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.72%. Comparing base (78482e5) to head (f1a6958).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #267   +/-   ##
=======================================
  Coverage   89.72%   89.72%           
=======================================
  Files          75       75           
  Lines        2812     2812           
  Branches      555      555           
=======================================
  Hits         2523     2523           
  Misses        289      289           

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