feat(llm): add Google Gemini API support (#230)

[at264939-ctrl]

Description

Resolves #230

This PR implements robust, first-class support for the Google Gemini API within the Langchain-Rust framework.

Key Features

• Core Implementations: Implemented the LLM trait (client.rs) alongside Builder patterns mirroring the existing architectural standards (OpenAI/Claude).
• Streaming (SSE): Added complete Server-Sent Events support for dynamic real-time token streaming.
• Embeddings: Implemented Embedder trait generating multi-dimensional context vectors natively via GeminiEmbedder.
• Error Handling: Eliminated native unwrap() panics in favor of explicitly bubbled contexts preventing silent system crashes.
• Type Safety: Strictly modeled message and configuration payload maps natively with optional fallbacks.
• Zero Warnings (CI-ready): Checked completely uniformly against workspace-wide cargo clippy -D warnings with optimal code normalization preventing compilation regressions.

Testing

• Fully equipped with localized unit tests ensuring parsing accuracy.
• Zero feature flag errors.