fix(run_agent): gate concurrent checkpoint preflight on block_result (fixes #34827)

In the concurrent tool-execution path, checkpoint preflight (write_file,
patch, destructive terminal) fired BEFORE plugin guardrail block_result
was computed. A blocked write_file could still dirty checkpoint state
(doc_modified_this_turn, _last_write_file_call_id, turn_counter).

Move checkpoint preflight to AFTER block_result computation, gated on
`if block_result is None:` — matching the invariant the sequential path
already enforces.

agent/tool_executor.py

@@ -180,28 +180,9 @@ def execute_tool_calls_concurrent(agent, assistant_message, messages: list, effe
         except Exception:
             pass
 
-        # Checkpoint for file-mutating tools
-        if function_name in {"write_file", "patch"} and agent._checkpoint_mgr.enabled:
-            try:
-                file_path = function_args.get("path", "")
-                if file_path:
-                    work_dir = agent._checkpoint_mgr.get_working_dir_for_path(file_path)
-                    agent._checkpoint_mgr.ensure_checkpoint(work_dir, f"before {function_name}")
-            except Exception:
-                pass
-
-        # Checkpoint before destructive terminal commands
-        if function_name == "terminal" and agent._checkpoint_mgr.enabled:
-            try:
-                cmd = function_args.get("command", "")
-                if _is_destructive_command(cmd):
-                    cwd = function_args.get("workdir") or os.getenv("TERMINAL_CWD", os.getcwd())
-                    agent._checkpoint_mgr.ensure_checkpoint(
-                        cwd, f"before terminal: {cmd[:60]}"
-                    )
-            except Exception:
-                pass
-
+        # ── Block evaluation (BEFORE checkpoint preflight) ───────────
+        # We must know whether the tool will execute before touching
+        # checkpoint state (dedup slot, real snapshots).
         block_result = None
         blocked_by_guardrail = False
         if _ts_scope_block is not None:
@@ -224,6 +205,30 @@ def execute_tool_calls_concurrent(agent, assistant_message, messages: list, effe
                     block_result = agent._guardrail_block_result(guardrail_decision)
                     blocked_by_guardrail = True
 
+        # ── Checkpoint preflight (only for tools that will execute) ──
+        if block_result is None:
+            # Checkpoint for file-mutating tools
+            if function_name in {"write_file", "patch"} and agent._checkpoint_mgr.enabled:
+                try:
+                    file_path = function_args.get("path", "")
+                    if file_path:
+                        work_dir = agent._checkpoint_mgr.get_working_dir_for_path(file_path)
+                        agent._checkpoint_mgr.ensure_checkpoint(work_dir, f"before {function_name}")
+                except Exception:
+                    pass
+
+            # Checkpoint before destructive terminal commands
+            if function_name == "terminal" and agent._checkpoint_mgr.enabled:
+                try:
+                    cmd = function_args.get("command", "")
+                    if _is_destructive_command(cmd):
+                        cwd = function_args.get("workdir") or os.getenv("TERMINAL_CWD", os.getcwd())
+                        agent._checkpoint_mgr.ensure_checkpoint(
+                            cwd, f"before terminal: {cmd[:60]}"
+                        )
+                except Exception:
+                    pass
+
         parsed_calls.append((tool_call, function_name, function_args, block_result, blocked_by_guardrail))
 
     # ── Logging / callbacks ──────────────────────────────────────────