Add agentic onboarding progress polling

[evanpurkhiser]

Stack 4 of 5. Previous: #121914. Next: #121916

Add the browser fallback for reading a complete onboarding snapshot by run ID. The same endpoint lets the browser cancel an active run while preserving the final state for rendering.

Reads and cancellation enforce the original user and organization ownership boundary.

[sentry]

Sentry Snapshot Testing

Name	Added	Removed	Changed	Renamed	Unchanged	Skipped	Status
sentry-frontend sentry-frontend	0	0	0	0	451	0	✅ Unchanged

⚙️ sentry-frontend Snapshot Settings

[sentry-warden]

Delete endpoint catches broad ValueError from deserialization failures

The broad except ValueError in delete() catches InvalidOnboardingRun from corrupt or incompatible Redis snapshots and returns a misleading terminal-run message instead of the actual error.

Evidence

• cancel() in OnboardingProgressService raises ValueError("Onboarding run is terminal") only when run_status is not ACTIVE.
• _atomic_update() calls _deserialize() first, which invokes OnboardingRun.from_dict(). Any schema mismatch or corrupt data raises InvalidOnboardingRun, which inherits from ValueError.
• The delete() handler catches all ValueError with CANNOT_CANCEL_RUN_DETAIL, so a deserialization failure is reported as "cannot be cancelled after it is terminal" instead of the actual data error.
• The get() endpoint on the same service does not guard ValueError from _deserialize, causing a 500 on malformed snapshots.

_{Identified by Warden · sentry-backend-bugs · DRG-BH3}