feat(core): add Gemini 1.5 embeddings support and green tests

[ali90h]

Adds a new Gemini15Embeddings class to wrap the Google Gemini 1.5 embeddings endpoint, with full test coverage and all CI checks passing.

Issue:
None

Dependencies:
None

[vercel]

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1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
langchain	⬜️ Ignored (Inspect)	Visit Preview		Aug 3, 2025 6:39am

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #32330 will not alter performance

_{Comparing ali90h:feature/gemini15-embeddings (e6f4602) with master (32e5040)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 13 untouched benchmarks

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32330 will not alter performance

_{Comparing ali90h:feature/gemini15-embeddings (e6f4602) with master (32e5040)}

Summary

✅ 14 untouched benchmarks