FN-5789: normalize missing sentence spaces in streamed deltas

Repair provider-agnostic streaming deltas so sentence boundaries keep expected spacing in chat and agent logs.

- add shared streaming-delta normalization helpers for text/thinking events
- apply normalization at executor and PI message_update chokepoints before emitting deltas
- add focused engine tests covering punctuation-boundary repairs and no-op cases
- add a patch changeset for @runfusion/fusion describing the fix

Files changed:
 .changeset/fn-5789-streaming-space-repair.md       |  5 ++
 .../engine/src/__tests__/streaming-delta.test.ts   | 84 ++++++++++++++++++++
 packages/engine/src/executor.ts                    | 11 ++-
 packages/engine/src/pi.ts                          | 13 +++-
 packages/engine/src/streaming-delta.ts             | 91 ++++++++++++++++++++++
 5 files changed, 197 insertions(+), 7 deletions(-)

Fusion-Task-Id: FN-5789

Fusion-Task-Lineage: e60ed2ae-c006-42f9-9ab0-f4e79560fb66

.changeset/fn-5789-streaming-space-repair.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Repair dropped spaces after sentence-ending punctuation in streamed agent responses (chat and agent logs) across all providers by applying the streaming-delta sentence-boundary fix at the shared engine delta chokepoints, not just the per-provider CLI bridges.