Make _from_exception a typed dataclass field; revert accidental snapshot changes

- Python: replace dynamic attribute with proper dataclass field
  (field(default=False, repr=False)) so type-checkers can see it
- Revert accidentally committed snapshot modifications for
  blob_attachments and message_attachments tests

Author: Morabbin

python/copilot/session.py

@@ -948,7 +948,7 @@ async def _execute_tool_and_respond(
             # sent via the top-level error param so the CLI formats them with its
             # standard "Failed to execute..." message. Deliberate user-returned
             # failures send the full structured result to preserve metadata.
-            if getattr(tool_result, "_from_exception", False):
+            if tool_result._from_exception:
                 await self.rpc.tools.handle_pending_tool_call(
                     SessionToolsHandlePendingToolCallParams(
                         request_id=request_id,

python/copilot/tools.py

@@ -10,7 +10,7 @@
 import inspect
 import json
 from collections.abc import Awaitable, Callable
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Any, Literal, TypeVar, get_type_hints, overload
 
 from pydantic import BaseModel
@@ -38,6 +38,7 @@ class ToolResult:
     binary_results_for_llm: list[ToolBinaryResult] | None = None
     session_log: str | None = None
     tool_telemetry: dict[str, Any] | None = None
+    _from_exception: bool = field(default=False, repr=False)
 
 
 @dataclass
@@ -194,20 +195,16 @@ async def wrapped_handler(invocation: ToolInvocation) -> ToolResult:
             except Exception as exc:
                 # Don't expose detailed error information to the LLM for security reasons.
                 # The actual error is stored in the 'error' field for debugging.
-                tr = ToolResult(
+                return ToolResult(
                     text_result_for_llm=(
                         "Invoking this tool produced an error. "
                         "Detailed information is not available."
                     ),
                     result_type="failure",
                     error=str(exc),
                     tool_telemetry={},
+                    _from_exception=True,
                 )
-                # Mark as exception-originated so _execute_tool_and_respond
-                # sends it via the top-level error param (matching CLI formatting)
-                # rather than as a structured result.
-                tr._from_exception = True  # type: ignore[attr-defined]
-                return tr
 
         return Tool(
             name=tool_name,

test/snapshots/session_config/should_accept_blob_attachments.yaml

@@ -5,3 +5,4 @@ conversations:
       - role: system
         content: ${system}
       - role: user
+        content: Describe this image

test/snapshots/session_config/should_accept_message_attachments.yaml

@@ -1,10 +1,6 @@
 models:
   - claude-sonnet-4.5
 conversations:
-  - messages:
-      - role: system
-        content: ${system}
-      - role: user
   - messages:
       - role: system
         content: ${system}