Add opt-in nonfatal handling for clock read failures (#45825)

## Why

Clock provider failures can abort a turn while preparing time context or running clock tools. Allow turns to continue with an explicit indication that the current time is unavailable.

## What changed

- Add `features.nonfatal_clock_read_errors`, disabled by default, to report clock failures to the model without failing the turn.
- Emit a generic `failed to read current time` notice for context reads and tool errors, without exposing provider error details. Deduplicate context notices per turn and compaction window, and remove inherited notices from forked subagent context.
- Omit unavailable environment dates and explicitly clear previously visible dates with `<current_date status="unavailable" />`.
- Return external sleep clock failures to the model when the feature is enabled, preserving sleep item completion notifications.

## Testing

Add coverage for continued inference after clock failures, notice deduplication across compaction, subagent notice filtering, environment date removal and recovery, and sleep failures during initial and polling reads.

GitOrigin-RevId: a39c3723c06a6f786d8ad59d667e8b9f626c973e

codex-rs/core/src/agent/control/spawn.rs

@@ -6,6 +6,7 @@ use crate::codex_thread::CodexThread;
 use crate::config::PermissionProfileSnapshot;
 use crate::context::ContextualUserFragment;
 use crate::context::CurrentTimeReminder;
+use crate::context::CurrentTimeUnavailable;
 use crate::context::DeveloperInstructions;
 use crate::context::GuardianContextMode;
 use crate::context::ManagedDeveloperInstructions;
@@ -135,6 +136,7 @@ fn retain_forked_developer_message(
                 ))
             || MultiAgentModeInstructions::matches_text(text)
             || CurrentTimeReminder::matches_text(text)
+            || CurrentTimeUnavailable::matches_text(text)
             || usage_hint_texts
                 .iter()
                 .any(|usage_hint_text| usage_hint_text == text))