Plumb format argument through MCP Orbit tool calls

What does this MR do and why?

Plumbs the format argument from MCP tool call arguments through to the GKG gRPC layer. Previously both query_graph and get_graph_schema hardcoded format: :llm. Now when an LLM passes format: "raw" in tool arguments, it receives structured JSON instead of LLM-optimized text.

Related

• Companion: gitlab-org/orbit/knowledge-graph!877 (merged)
• Relates to gitlab-org/orbit/knowledge-graph#271 (closed)
• Relates to gitlab-org/orbit/knowledge-graph#433 (closed)
• Relates to gitlab-org#20566 (closed)

How to validate locally

# With GKG running, call get_graph_schema with raw format:
POST /api/v4/orbit/mcp
{"jsonrpc":"2.0","method":"tools/call","id":"1","params":{"name":"get_graph_schema","arguments":{"format":"raw"}}}
# => structured JSON instead of TOON text

# Without format (default behavior unchanged):
POST /api/v4/orbit/mcp
{"jsonrpc":"2.0","method":"tools/call","id":"1","params":{"name":"get_graph_schema","arguments":{}}}
# => TOON text (same as before)