fix(api): Support stage-specific onboarding metadata

[evanpurkhiser]

Sentry onboarding runs now store project slugs and verification issue IDs in stage-specific extra objects. This updates the API schemas, outgoing status payloads, response parsing, and mocks to follow the contract introduced by getsentry/sentry#122076.

The onboarding_status_update tool now accepts a nested update object whose schema discriminates on stage. Callers must move stage, status, runStatus, and eventNote under update, and send projectSlugs or issueIds through that stage's extra object.

[cursor]

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