[app-server] fix: emit item/fileChange/outputDelta for file change items

[owenlin0]

Emit item/fileChange/outputDelta instead of item/execCommand/outputDelta for FileChange items.

The underlying EventMsg::ExecCommandOutputDelta is emitted for apply_patch, shell, and unified exec, and since we represent apply_patch calls as FileChange items, we were seeing these events:

item/started <- FileChange
item/execCommand/outputDelta <- our code assumed that EventMsg::ExecCommandOutputDelta is always for the CommandExecution item
item/completed <- FileChange

With this fix we'll have these events:

item/started <- FileChange
item/fileChange/outputDelta <- our code assumed that EventMsg::ExecCommandOutputDelta is always for the CommandExecution item
item/completed <- FileChange