Return Orbit MCP client errors as tool results

What does this MR do and why?

The Orbit MCP endpoint turned every gRPC error from Orbit into JSON-RPC -32603 "Internal error". When an agent sent a bad command name, it did not see the Orbit error, such as Tool not found: no_such_command. Now a client error comes back as an isError tool result with the Orbit error text. Client errors are the codes INVALID_ARGUMENT, NOT_FOUND, and FAILED_PRECONDITION. Other errors still return the generic error.

References

• Relates to gitlab-org/orbit/knowledge-graph#1377. Companion MRs: gitlab-org/orbit/knowledge-graph!2770 (merged) and gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!7405.
• call_tool.rb#L23-27 and grpc_client.rb#L29-46.
• The ConnectionError#client_error? helper follows the same approach as !261207 (merged).

Screenshots or screen recordings

Before	After
{"jsonrpc":"2.0","error":{"code":-32603,"message":"Internal error"},"id":3}

How to set up and validate locally

1. Run GDK with Orbit and Duo Agent Platform.
2. Call the MCP endpoint with an unknown command:

   curl -sk "https://gdk.test:3443/api/v4/orbit/mcp" \
     -H "PRIVATE-TOKEN: $TOKEN" -H "Content-Type: application/json" \
     -H "Accept: application/json, text/event-stream" \
     -d '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"invoke_command","arguments":{"command_name":"no_such_command","parameters":{}}}}'

3. The response is {"jsonrpc":"2.0","result":{"content":[{"type":"text","text":"Tool not found: no_such_command"}],"isError":true},"id":3}.
4. A valid query_graph call still returns isError: false. If Orbit is down, the response is still -32603.

Specs: call_tool_spec.rb, grpc_client_spec.rb, and the Orbit MCP request spec ran 274 examples with 0 failures. Before the fix, the new examples failed.

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