Enhance error handling in RPC functions for clearer messages

[pablomendezroyo]

• Add toRpcErrorMessage() helper on the backend to always produce a string error message
• Add toErrorMessage() helper on the client to extract human-readable messages from JSON-RPC error responses
• Fix JSON-RPC error codes: use -32601 specifically for method-not-found, and -32000 for all other caught server errors

[Marketen]

@copilot review this PR

[Copilot]

@copilot review this PR

Here's the review of the PR changes (commit 253b2f6):

packages/brain/src/modules/apiServers/ui/startUiServer.ts

Added toRpcErrorMessage(error: unknown): string on the backend. This ensures the JSON-RPC 2.0 message field is always a string (the spec requires it). It correctly handles null/undefined, plain strings, Error instances, and arbitrary objects via JSON.stringify with a String() fallback.

One pre-existing note (not introduced by this PR): the error code -32601 is the JSON-RPC 2.0 "Method Not Found" code but is used here for all caught errors. The correct code for generic server errors is -32000 to -32099. This was in the code before these changes, but worth tracking as a follow-up.

packages/ui/src/socket/socket.ts

Added toErrorMessage(value: unknown): string on the client. It handles the JSON-RPC { code, message } shape and also the legacy case where the backend sent a raw error object as message (before this fix). The raw error payload is preserved on the thrown Error as .rpcError which is handy for debugging.

CI: ✅ Build and tests passed (22611466384).

Overall the changes are correct, well-scoped, and improve spec-compliance. The only suggestion for a follow-up is correcting the error code from -32601 to a server-error range code (e.g. -32000) for non-"method not found" failures.

[Marketen]

@copilot correct the error code from -32601 to a server-error range code (e.g. -32000) for non-"method not found" failures.

[Copilot]

@copilot correct the error code from -32601 to a server-error range code (e.g. -32000) for non-"method not found" failures.

Done in 4a7bc07. The method-not-found check is now separated from the try/catch so each path uses the correct code:

• -32601 when the requested method doesn't exist in rpcMethods
• -32000 for any other runtime error caught during method execution