fix: standardize "Session not found" error messages in CodeExecutionPlugin

[fuseraft]

src/Infrastructure/Plugins/CodeExecutionPlugin.cs has three "session not found" errors with inconsistent copy:

Line	Message
141	"Session '{sessionId}' not found. Use repl_start to create one."
194	"Session '{sessionId}' not found."
220	"Session '{sessionId}' not found."

Only the first error gives the user actionable guidance. All three code paths should give the same hint so the experience is consistent.

Suggested fix: Update lines 194 and 220 to append " Use repl_start to create one.".

Files to touch: src/Infrastructure/Plugins/CodeExecutionPlugin.cs