feat(onboarding): Model agentic setup progress

[evanpurkhiser]

Stack 1 of 5. Next: #121913

Define the ordered onboarding stages and their durable state transitions. Later stages complete earlier required work, optional stages remain explicit, and completed stages never regress.

The run model keeps terminal state separate from stage state and validates concise UI notes and failure reasons before they reach storage.

[sentry-warden]

KeyError on unknown stage in validate_update instead of ValueError

STAGE_DEFINITION_BY_STAGE[update.stage] crashes with KeyError when the stage string is not registered. Use .get() and raise ValueError so callers receive a clean validation error instead of a raw dict miss.

Evidence

• STAGE_DEFINITION_BY_STAGE only contains keys built from STAGE_DEFINITIONS.
• ProgressUpdate.stage is typed as Stage, but Python dataclasses do not enforce enum membership at runtime, so a caller can pass any string (e.g., deserializing JSON directly into the dataclass).
• When an unregistered stage reaches validate_update, the direct lookup STAGE_DEFINITION_BY_STAGE[update.stage] raises KeyError instead of a controlled ValueError.
• This matches the production HANDLERS[event_type] pattern: registry dict lookups without existence checks crash on unexpected input.

Also found at 1 additional location

• src/sentry/onboarding/agentic_progress/model.py:269-297

_{Identified by Warden · sentry-backend-bugs · 43T-LWE}