fix(cron): accept bare cron-expression string in Schedule deserializer (Sentry CORE-RUST-FY)

[M3gA-Mind]

Summary

Rebased version of #2868 by @graycyrus on the latest upstream/main.

The original PR's branch included a cherry-picked observability commit (fix(observability): classify Embedding API 401 Invalid-token as SessionExpired) that was independently merged to main as PR #2869. This caused a merge conflict (mergeable_state: dirty) that blocked CI from running.

This PR contains only the two cron-fix commits from #2868, cleanly applied on top of current main:

1. fix(cron): accept bare cron-expression string in Schedule deserializer
2. style(cron): apply cargo fmt to Schedule deserializer

No functional changes from #2868. The fix and tests are identical.

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Root cause (Sentry CORE-RUST-FY / #2866): openhuman.cron_update deserializes schedule as an internally-tagged Schedule enum. Agents and some frontend callers send a bare string like "0 9 * * 1" instead of {"kind":"cron","expr":"0 9 * * 1"}. Serde rejected this with: invalid type: string, expected internally tagged enum Schedule.

Fix: Custom Deserialize impl for Schedule in src/openhuman/cron/types.rs. A bare string → Schedule::Cron { expr, tz: None, active_hours: None }. The full object form is handled by visit_map delegating to an inner #[derive(Deserialize)] tagged enum via MapAccessDeserializer. Serialization unchanged.

Tests: 4 unit tests — bare string, 5-field */5, CronJobPatch with bare string (exact failing payload), regression guard for the structured object form.

Closes #2868.

Test plan

• cargo test --lib openhuman::cron::types — all 20 pass
• cargo check — clean
• Diff coverage ≥ 80% — all new branches exercised by the 4 tests

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Reviewing files that changed from the base of the PR and between 94f8e4e and d92a7bd.

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• src/openhuman/cron/types.rs

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