refactor(runtime): drop unused receiver from handleStream

handleStream is the chunked-stream aggregator that turns an LLM
provider's chat.MessageStream into a streamResult plus a sequence
of events. It was declared as a method on *LocalRuntime, but its
body never touches `r` — the receiver is purely an artefact of
where the file sits. Drop it.

This is a pure isolation win, not a behaviour change:

- handleStream is now a free function, callable (and unit-testable)
  without spinning up a LocalRuntime, a team, hooks, or anything
  else; a mock chat.MessageStream is the only dependency.
- The events parameter tightens from `chan Event` to `chan<- Event`,
  matching the rest of the package's send-only conventions and
  documenting that the function never reads from the channel.
- Doc comment now states the dependency direction explicitly: the
  loop calls into the chunker, never the reverse.
- Only caller (tryModelWithFallback in fallback.go) updated to drop
  the `r.` qualifier.

streamResult and stripImageContent are unchanged. A future
follow-up could lift this into pkg/runtime/internal/streamproc to
enforce the boundary at compile time, but that requires inverting
the events-channel dependency (Event is currently a runtime type)
and is out of scope here.

Assisted-By: docker-agent

Author: dgageot

pkg/runtime/fallback.go

@@ -285,7 +285,7 @@ func (r *LocalRuntime) tryModelWithFallback(
 				}
 			}
 
-			res, err := r.handleStream(ctx, stream, a, agentTools, sess, m, events)
+			res, err := handleStream(ctx, stream, a, agentTools, sess, m, events)
 			if err != nil {
 				lastErr = err
 

pkg/runtime/streaming.go

@@ -34,7 +34,13 @@ type streamResult struct {
 // events (content deltas, partial tool calls, reasoning tokens) and returning
 // the aggregated streamResult. The caller is responsible for adding the
 // resulting assistant message to the session.
-func (r *LocalRuntime) handleStream(ctx context.Context, stream chat.MessageStream, a *agent.Agent, agentTools []tools.Tool, sess *session.Session, m *modelsdev.Model, events chan Event) (streamResult, error) {
+//
+// handleStream is a pure stream-aggregation routine: it does not touch
+// runtime state and can be unit-tested by feeding a mock chat.MessageStream.
+// It is intentionally a free function rather than a method on *LocalRuntime
+// so the dependency direction is explicit (the loop calls into the chunker,
+// never the reverse).
+func handleStream(ctx context.Context, stream chat.MessageStream, a *agent.Agent, agentTools []tools.Tool, sess *session.Session, m *modelsdev.Model, events chan<- Event) (streamResult, error) {
 	defer stream.Close()
 
 	var fullContent strings.Builder