fix(gateway): replace os.environ session state with contextvars for concurrency safety

When two gateway messages arrived concurrently, _set_session_env wrote
HERMES_SESSION_PLATFORM/CHAT_ID/CHAT_NAME/THREAD_ID into the process-global
os.environ. Because asyncio tasks share the same process, Message B would
overwrite Message A's values mid-flight, causing background-task notifications
and tool calls to route to the wrong thread/chat.

Replace os.environ with Python's contextvars.ContextVar. Each asyncio task
(and any run_in_executor thread it spawns) gets its own copy, so concurrent
messages never interfere.

Changes:
- New gateway/session_context.py with ContextVar definitions, set/clear/get
  helpers, and os.environ fallback for CLI/cron/test backward compatibility
- gateway/run.py: _set_session_env returns reset tokens, _clear_session_env
  accepts them for proper cleanup in finally blocks
- All tool consumers updated: cronjob_tools, send_message_tool, skills_tool,
  terminal_tool (both notify_on_complete AND check_interval blocks), tts_tool,
  agent/skill_utils, agent/prompt_builder
- Tests updated for new contextvar-based API

Fixes #7358

Co-authored-by: teknium1 <127238744+teknium1@users.noreply.github.com>

gateway/run.py

@@ -2442,8 +2442,8 @@ class GatewayRunner:
         # Build session context
         context = build_session_context(source, self.config, session_entry)
         
-        # Set environment variables for tools
-        self._set_session_env(context)
+        # Set session context variables for tools (task-local, concurrency-safe)
+        _session_env_tokens = self._set_session_env(context)
         
         # Read privacy.redact_pii from config (re-read per message)
         _redact_pii = False
@@ -3276,8 +3276,8 @@ class GatewayRunner:
                 "Try again or use /reset to start a fresh session."
             )
         finally:
-            # Clear session env
-            self._clear_session_env()
+            # Restore session context variables to their pre-handler state
+            self._clear_session_env(_session_env_tokens)
     
     def _format_session_info(self) -> str:
         """Resolve current model config and return a formatted info block.
@@ -6176,20 +6176,27 @@ class GatewayRunner:
 
         return True
 
-    def _set_session_env(self, context: SessionContext) -> None:
-        """Set environment variables for the current session."""
-        os.environ["HERMES_SESSION_PLATFORM"] = context.source.platform.value
-        os.environ["HERMES_SESSION_CHAT_ID"] = context.source.chat_id
-        if context.source.chat_name:
-            os.environ["HERMES_SESSION_CHAT_NAME"] = context.source.chat_name
-        if context.source.thread_id:
-            os.environ["HERMES_SESSION_THREAD_ID"] = str(context.source.thread_id)
-    
-    def _clear_session_env(self) -> None:
-        """Clear session environment variables."""
-        for var in ["HERMES_SESSION_PLATFORM", "HERMES_SESSION_CHAT_ID", "HERMES_SESSION_CHAT_NAME", "HERMES_SESSION_THREAD_ID"]:
-            if var in os.environ:
-                del os.environ[var]
+    def _set_session_env(self, context: SessionContext) -> list:
+        """Set session context variables for the current async task.
+
+        Uses ``contextvars`` instead of ``os.environ`` so that concurrent
+        gateway messages cannot overwrite each other's session state.
+
+        Returns a list of reset tokens; pass them to ``_clear_session_env``
+        in a ``finally`` block.
+        """
+        from gateway.session_context import set_session_vars
+        return set_session_vars(
+            platform=context.source.platform.value,
+            chat_id=context.source.chat_id,
+            chat_name=context.source.chat_name or "",
+            thread_id=str(context.source.thread_id) if context.source.thread_id else "",
+        )
+
+    def _clear_session_env(self, tokens: list) -> None:
+        """Restore session context variables to their pre-handler values."""
+        from gateway.session_context import clear_session_vars
+        clear_session_vars(tokens)
     
     async def _enrich_message_with_vision(
         self,