fix: preserve Google embedding usage metadata

[mikehostetler]

Summary

• preserve Google embedding usageMetadata when normalizing embedding responses
• restore return_usage: true output and [:req_llm, :token_usage] telemetry for standard Google embeddings
• add provider-level and public API coverage for the embedding usage path

Closes #482.

Verification

• mix test test/req_llm/embedding_test.exs
• mix test test/providers/google_test.exs
• mix test
• mix quality