[Rails] Add LLM Codebase Semantic Search Tool

Context

Add the Codebase Semantic Search Tool.

This tool will make use of the Codebase Embeddings Search Service introduced in [Rails] Add Codebase Embeddings Search Service (#537896 - closed).

References

• Adding a new Duo Chat tool documentation
• Duo Chat process flow
• Duo Chat tools
• Blueprint: Add blueprint for Codebase as Chat Context (gitlab-com/content-sites/handbook!12792 - merged)
• POC: Draft: POC: Codebase as Duo Chat Context (!189573 - closed)

Proposal

• the semantic search tool should make use of the Codebase Embeddings Search Service
• the semantic search result will then be added as content to the repository additional context
• the Gitlab::Llm::Completions::Chat class should then call the Semantic Search Tool to enhance the additional context

POC: Draft: POC: Codebase as Duo Chat Context (!189573 - closed)