fix(prompt): isolate truncation warnings per context

Follow-up to salvaged PR #41619: replace the module-global
_truncation_warnings list with a contextvars.ContextVar so concurrent
gateway-session prompt builds can't drain or clear each other's pending
warnings (cross-session leak). Adds a context-isolation test.

agent/prompt_builder.py

@@ -8,6 +8,7 @@ import json
 import logging
 import os
 import threading
+import contextvars
 from collections import OrderedDict
 from pathlib import Path
 
@@ -976,14 +977,32 @@ def _get_context_file_max_chars() -> int:
     return CONTEXT_FILE_MAX_CHARS
 
 # Collect truncation warnings so the caller (run_agent) can surface them.
-_truncation_warnings: list = []
+# A ContextVar (not a module-global list) isolates accumulation per thread /
+# per async task, so concurrent gateway-session prompt builds can't drain or
+# clear each other's pending warnings (cross-session leak). Each build runs in
+# its own context, collects its own warnings, and drains them synchronously.
+_truncation_warnings: "contextvars.ContextVar[Optional[list]]" = contextvars.ContextVar(
+    "context_file_truncation_warnings", default=None
+)
+
+
+def _record_truncation_warning(msg: str) -> None:
+    """Append a truncation warning to the current context's accumulator."""
+    warnings = _truncation_warnings.get()
+    if warnings is None:
+        warnings = []
+        _truncation_warnings.set(warnings)
+    warnings.append(msg)
 
 
 def drain_truncation_warnings() -> list:
-    """Return and clear any truncation warnings accumulated since last drain."""
-    warnings = _truncation_warnings.copy()
-    _truncation_warnings.clear()
-    return warnings
+    """Return and clear any truncation warnings accumulated in this context."""
+    warnings = _truncation_warnings.get()
+    if not warnings:
+        return []
+    drained = list(warnings)
+    warnings.clear()
+    return drained
 
 
 # =========================================================================
@@ -1503,7 +1522,7 @@ def _truncate_content(content: str, filename: str, max_chars: Optional[int] = No
         f"increase context_file_max_chars or trim the file!"
     )
     logger.warning(msg)
-    _truncation_warnings.append(msg)
+    _record_truncation_warning(msg)
     head_chars = int(max_chars * CONTEXT_TRUNCATE_HEAD_RATIO)
     tail_chars = int(max_chars * CONTEXT_TRUNCATE_TAIL_RATIO)
     head = content[:head_chars]