refactor(chat): add streaming/shared/ package for StreamResult and utils

Foundation layer for the parallel refactor of stream_new_chat.py.
Extracts the StreamResult dataclass (tracks per-turn streaming state)
and a small set of shared utilities (resume_step_prefix, safe_float).

Add-only; no existing code imports from this package yet. Existing
stream_new_chat.py keeps its inline equivalents until cutover.

surfsense_backend/app/tasks/chat/streaming/shared/utils.py

@@ -0,0 +1,27 @@
+"""Small utilities used by streaming orchestrators and phases."""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+def resume_step_prefix(turn_id: str) -> str:
+    """Per-turn ``step_prefix`` for resume invocations.
+
+    Each ``stream_agent_events`` call constructs a fresh
+    ``AgentEventRelayState`` with ``thinking_step_counter=0``, so two consecutive
+    resume turns would otherwise both emit ``thinking-resume-1``, ``-2`` etc.
+    The frontend rehydrates ``currentThinkingSteps`` from the immediate prior
+    assistant message at the start of every resume — if the new stream's IDs
+    collide with the seeded ones, React renders sibling Timeline rows with the
+    same key. Salting with ``turn_id`` guarantees disjoint IDs across resumes
+    within one thread.
+    """
+    return f"thinking-resume-{turn_id}"
+
+
+def safe_float(value: Any, default: float = 0.0) -> float:
+    try:
+        return float(value)
+    except (TypeError, ValueError):
+        return default