test(web): drop idx_sessions_effective_activity before emulating a pre-column store

The salvaged #95487 adds an expression index over sessions.last_activity_at.
Two web_server tests emulate a legacy store with ALTER TABLE ... DROP COLUMN
last_activity_at, which SQLite refuses while an index references the
column ('error in index ... after drop column'). Drop the index first —
the same adjustment the PR already made in test_schema_read_probe.py. A
real pre-column store has neither the column nor the index, so the healed
path under test is unchanged.

tests/hermes_cli/test_web_server.py

@@ -440,6 +440,9 @@ class TestWebServerEndpoints:
 
         legacy = sqlite3.connect(str(db_path))
         try:
+            # SQLite refuses DROP COLUMN while an index references the
+            # column; a pre-column legacy store has neither.
+            legacy.execute("DROP INDEX IF EXISTS idx_sessions_effective_activity")
             legacy.execute(f"ALTER TABLE sessions DROP COLUMN {missing_column}")
             legacy.commit()
         finally:
@@ -486,6 +489,7 @@ class TestWebServerEndpoints:
 
         legacy = sqlite3.connect(str(db_path))
         try:
+            legacy.execute("DROP INDEX IF EXISTS idx_sessions_effective_activity")
             legacy.execute("ALTER TABLE sessions DROP COLUMN last_activity_at")
             legacy.commit()
         finally: