Add input validation to RPC command handler

[m-aebrer]

Add input validation to RPC command handler

The RPC command handler in rpc-mode.ts does not validate incoming JSONL commands before processing them. When a client sends a malformed command (e.g., {"type":"prompt","content":"..."} instead of the required {"type":"prompt","message":"..."}), the undefined field propagates into agent-session.ts and crashes with an opaque error:

Cannot read properties of undefined (reading 'startsWith')

Current Behavior

• No schema validation on incoming RPC commands
• Missing/misspelled fields pass through as undefined
• Crashes surface as internal JS errors rather than actionable RPC error responses

Proposed Behavior

• Validate required fields before dispatching commands (at minimum: message on prompt, steer, and follow_up)
• Return a clear RPC error response for invalid commands, e.g.:

  {"type":"response","command":"prompt","success":false,"error":"Missing required field 'message' in prompt command"}

• Never let undefined reach session.prompt()

Relevant Files

• packages/coding-agent/src/modes/rpc/rpc-mode.ts — handleCommand() dispatch
• packages/coding-agent/src/modes/rpc/rpc-types.ts — RpcCommand type definitions

Context

Discovered via issue 253 — a client sent content instead of message, which crashed the agent instead of returning a validation error. Low priority since the fix is on the client side, but better error messages would save debugging time.