AdvancedSQLiteSession.store_run_usage() records a phantom usage row for turn 0 when the current branch has no turn #4821

Summary

On an empty session, or on a branch whose turns were all popped, store_run_usage() inserts a turn_usage row for user_turn_number = 0. get_session_usage() then reports total_turns: 1 while get_items() is []. test_empty_session_operations and the method docstrings say usage is never recorded against a turn that has no rows.

Affected code (main @ 89c02c8), src/agents/extensions/memory/advanced_sqlite_session.py

• store_run_usage (:618-655) calls _capture_current_turn (:657-686), which on a branch without turns returns user_turn_number = 0 (COALESCE(MAX(...), 0)) and turn_anchor = None (MIN(id) of no rows).
• _update_turn_usage_internal (:1883-1970) only runs the removed-turn guard under if turn_anchor is not None:, so a None anchor skips the guard and the INSERT OR REPLACE lands.

Reproduction

from unittest.mock import MagicMock
from agents import RunResult, Usage
from agents.extensions.memory import AdvancedSQLiteSession

s = AdvancedSQLiteSession(session_id="x", create_tables=True)
r = MagicMock(spec=RunResult)
r.context_wrapper = MagicMock()
r.context_wrapper.usage = Usage(requests=1, input_tokens=10, output_tokens=5, total_tokens=15)
await s.store_run_usage(r)
print(await s.get_items())          # []
print(await s.get_session_usage())  # {'requests': 1, ..., 'total_turns': 1}   expected: None
print(await s.get_turn_usage())     # [{'user_turn_number': 0, ...}]           expected: []

The same happens after add_items followed by pop_item until the branch is empty. A later real turn plus store_run_usage(requests=2) then reports requests: 3, total_turns: 2 instead of requests: 2, total_turns: 1.

Root cause

A None anchor means "no turn on this branch", but _update_turn_usage_internal treats it as "no guard requested" and writes unconditionally.