Store agentic onboarding metadata by stage

[evanpurkhiser]

Agentic onboarding progress now stores structured metadata on the stage that owns it. The status endpoint accepts a stage-specific context object, and run snapshots return that context alongside each stage instead of exposing project slugs and issue IDs at the run level.

Context models use Pydantic for strict runtime validation at both the API and Redis persistence boundaries while generic stage definitions preserve the corresponding Python context type. Consumers of projectSlugs and issueIds must read or write them through the create_project and receive_verification_error stage contexts.

[sentry-warden]

KeyError when deserializing stage without a definition

Deserializing persisted onboarding data with a valid Stage enum value that lacks a STAGE_DEFINITIONS entry raises an uncaught KeyError instead of being skipped gracefully.

Evidence

• from_dict calls _deserialize_extra(stage, item.extra) immediately after Stage(item.stage) succeeds.
• _deserialize_extra calls get_stage_definition(stage), which does STAGE_DEFINITION_BY_STAGE[stage] without checking existence first.
• A stage kept in the Stage enum but removed from STAGE_DEFINITIONS would pass the except ValueError guard and then crash on this dict access.
• The outer except in from_dict only catches TypeError, ValidationError, and ValueError, so KeyError propagates uncaught.
• Previously from_dict did not depend on STAGE_DEFINITION_BY_STAGE; this dependency was introduced by the new _deserialize_extra call in this change.

_{Identified by Warden · sentry-backend-bugs · HBZ-292}