fix: gateway token double-counting — use absolute set instead of increment (#3317)

The gateway's update_session() used += for token counts, but the cached
agent's session_prompt_tokens / session_completion_tokens are cumulative
totals that grow across messages. Each update_session call re-added the
running total, inflating usage stats with every message (1.7x after 3
messages, worse over longer conversations).

Fix: change += to = for in-memory entry fields, add set_token_counts()
to SessionDB that uses direct assignment instead of SQL increment, and
switch the gateway to call it.

CLI mode continues using update_token_counts() (increment) since it
tracks per-API-call deltas — that path is unchanged.

Based on analysis from PR #3222 by @zaycruz (closed).

Co-authored-by: zaycruz <zay@users.noreply.github.com>

gateway/session.py

@@ -785,7 +785,7 @@ class SessionStore:
 
         if self._db and db_session_id:
             try:
-                self._db.update_token_counts(
+                self._db.set_token_counts(
                     db_session_id,
                     input_tokens=input_tokens,
                     output_tokens=output_tokens,