[Tech Debt] Handle Errors correctly in the ext proc server

[maleck13]

Jira: https://issues.redhat.com/browse/CONNLINK-812

what

In the POC error handling was not a priority. This has led to several places where error handling is not ideal. We should have a clear pattern for handling errors in the ext_proc process method and understand what returning an error here will mean to the client and envoy

https://github.com/kagenti/mcp-gateway/blob/main/internal/mcp-router/server.go#L73 as an example of this