fix(provider): ignore SSE comment heartbeats for chunk timeout - #43607

…ray semantics

Deviations from the ported upstream hunks, per review:
- anomalyco#43881: the empty-stream guard now fails only when the attempt produced
  no text/reasoning delta and no tool-call event. Providers may stream real
  content but omit the usage block and finish reason; retrying those would
  duplicate already-persisted output (up to 6 attempts). Empty-content
  deltas still count as empty, so the original clean-EOF retry is intact.
- anomalyco#38939: listeners are an Array again. The Set container silently
  deduplicated identical callback registrations and removed every entry on
  the first unsubscribe; duplicates must deliver independently.
- adds the missing multi-byte UTF-8 split regression for anomalyco#43607's
  streaming TextDecoder

…co#42150 anomalyco#42176 anomalyco#43881 anomalyco#43607)

- O(N) text/reasoning delta accumulation instead of O(N^2) string
  concat (anomalyco#42150) — the lazy chunk buffer joins on read
- finish reason 'error' is set when a stream fails mid-flight (anomalyco#42176)
- clean-EOF empty provider streams retry like transient errors (anomalyco#43881),
  narrowed from the upstream patch: only an attempt that produced no
  text/reasoning delta and no tool call qualifies, so providers that
  stream content but omit usage/finish are not retried into duplicate
  output
- SSE comment heartbeats no longer reset the chunk timeout (anomalyco#43607);
  the streaming TextDecoder handles multi-byte characters split across
  reads (regression covered)