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>

tools/tts_tool.py

@@ -480,7 +480,8 @@ def text_to_speech_tool(
     # Telegram voice bubbles require Opus (.ogg); OpenAI and ElevenLabs can
     # produce Opus natively (no ffmpeg needed).  Edge TTS always outputs MP3
     # and needs ffmpeg for conversion.
-    platform = os.getenv("HERMES_SESSION_PLATFORM", "").lower()
+    from gateway.session_context import get_session_env
+    platform = get_session_env("HERMES_SESSION_PLATFORM", "").lower()
     want_opus = (platform == "telegram")
 
     # Determine output path