Make environment context date and timezone configurable (#52164)

## What changed

Add `include_environment_context_time`, enabled by default. Setting it to `false` omits `<current_date>` and `<timezone>` while retaining the rest of the environment context, and skips the clock read used to populate the date.

## Testing

Extend configuration, session context, and external time provider tests to cover enabled and disabled time fields, context updates across turns, and current-time reminder behavior.

GitOrigin-RevId: 5071ce1701c044b680624d0cdbba7e0bc2b5bbe6

codex-rs/core/src/context/world_state/environment.rs

@@ -61,7 +61,10 @@ impl EnvironmentsState {
             environments: environment_states(environments),
             shell_version,
             current_date,
-            timezone: turn_context.timezone.clone(),
+            timezone: turn_context
+                .timezone
+                .clone()
+                .filter(|_| turn_context.config.include_environment_context_time),
             network: network_from_turn_context(turn_context),
             filesystem: environments.primary().map(|environment| {
                 FileSystemContext::from_permission_profile(