Add agentic onboarding progress endpoints

[evanpurkhiser]

Stack 3 of 5. Previous: #121913. Next: #121915

Expose private organization endpoints for registering a browser run and accepting stage updates from the MCP tool. Requests require org:read access and bind every run to both the authenticated user and organization.

The API returns the complete normalized snapshot after each update so callers can observe accepted state and stop after a terminal run.

[sentry]

Sentry Snapshot Testing

Name	Added	Removed	Changed	Renamed	Unchanged	Skipped	Status
sentry-frontend sentry-frontend	-	-	-	-	-	-	⏳ Processing

⚙️ sentry-frontend Snapshot Settings

[sentry-warden]

Bare assert used to validate authenticated user presence

Replace assert user_id is not None with an explicit guard that returns a 401/400 for requests that lack a user (e.g. org auth tokens), since OrganizationPermission allows token-based authentication where request.user.id can be None.

Evidence

• AgenticOnboardingPermission inherits OrganizationPermission, whose determine_access method grants access for token auth (e.g. org auth tokens) even when request.user is anonymous.
• convert_args in OrganizationEndpoint already calls organization_service.get_organization_by_id(..., user_id=None) for such requests, confirming the base class tolerates a missing user.
• The assert user_id is not None in the view method will therefore raise AssertionError (500) instead of a proper client error when a valid org auth token with org:read hits this endpoint.

Also found at 1 additional location

• src/sentry/api/endpoints/organization_agentic_onboarding.py:132

_{Identified by Warden · sentry-backend-bugs · V5T-YSR}