fix(agent): propagate ContextVars to concurrent tool worker threads (#18123)

Propagates ContextVars (notably `tools.approval._approval_session_key`) into concurrent tool worker threads via `copy_context().run` — mirrors `asyncio.to_thread` semantics.

Fixes approval-card cross-session misrouting in concurrent gateway traffic. Repro'd on Slack: session A's dangerous-command approval was delivered to channel B (@syahidfrd).

Salvages #16660 — core 4-LOC fix preserved, unrelated `tests/eval_018/` scope contamination dropped. Adds 5 regression guards including an AST-level source check on the real call site.

Closes #16660.

Co-authored-by: firefly <promptsiren@gmail.com>
Co-authored-by: banditburai <banditburai@users.noreply.github.com>

run_agent.py

@@ -23,6 +23,7 @@ Usage:
 import asyncio
 import base64
 import concurrent.futures
+import contextvars
 import copy
 import hashlib
 import json
@@ -9443,7 +9444,9 @@ class AIAgent:
             with concurrent.futures.ThreadPoolExecutor(max_workers=max_workers) as executor:
                 futures = []
                 for i, (tc, name, args) in enumerate(parsed_calls):
-                    f = executor.submit(_run_tool, i, tc, name, args)
+                    # Propagate ContextVars (e.g. _approval_session_key); mirrors asyncio.to_thread.
+                    ctx = contextvars.copy_context()
+                    f = executor.submit(ctx.run, _run_tool, i, tc, name, args)
                     futures.append(f)
 
                 # Wait for all to complete with periodic heartbeats so the