fix(server): notify watchers after atomic policy commits

[NaveCohenMonday]

Summary

Notify sandbox watchers after an atomic UpdateConfig policy revision commits.
Without this notification, live WatchSandbox(follow_status=true) consumers can
continue showing stale sandbox state until another event occurs. Sandbox
supervisors poll GetSandboxConfig independently and are not affected by this
notification gap.

Related Issue

Fixes#2517

Changes

• Notify the sandbox watch bus only after put_policy_revision_atomic succeeds.
• Extend the existing same-policy/new-provenance regression test to assert that
  one committed revision emits exactly one watcher notification.
• Preserve conflict retry and failure behavior so unsuccessful writes do not
  notify observers.

Testing

• CARGO_INCREMENTAL=0 mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not applicable)

Focused test:

CARGO_INCREMENTAL=0 mise exec -- cargo test -p openshell-server \
update_config_same_policy_hash_with_new_provenance_creates_revision --lib
test result: ok. 1 passed; 0 failed

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable)

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[github-actions]

All contributors have signed the DCO ✍️ ✅
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[NaveCohenMonday]

I have read the DCO document and I hereby sign the DCO.

[drew]

gator-agent

PR Review Status

Validation: This concentrated server fix implements the confirmed watcher-notification defect in linked issue #2517 and stays within that issue's scope.
Head SHA: 28702360e622a60670c967098519e6ebb0185332

Review findings:

• No blocking findings remain. The notification is reached only after the atomic policy revision transaction commits, while failed writes, exhausted retries, and idempotent no-op requests return before it.
• The regression test covers delivery and exactly-once behavior for the affected same-policy/new-provenance path.

Docs: Not needed; this restores an existing internal watch notification invariant without changing a public API, CLI, configuration field, or documented workflow.

E2E: No test:* label is needed; the focused server unit test directly covers this in-memory notification hook.

Next state: gator:watch-pipeline

[drew]

/ok to test 2870236