[Agentic Chat Integration] Update name and description of the Codebase Search Tool

Context

From #572264 (comment 2802764249):

• update the tool name: get_code_context might not be very obvious for the LLM. We can use something like codebase_semantic_search
• update the tool description to give more information on its usage/purpose

Additional TODOs:

• The documentation must also be updated, see MR introducing documentation: Add `get_code_context` tool to MCP sever docs (!207617 - merged)
• Consider renaming the search_term parameter
• Update the descriptions for each parameter

References

TBA

Proposal

TBA