fix(schema): Allow missing projectSlugs/issueIds in AgenticOnboardingRunSchema

[sentry]

This PR addresses issue MCP-SERVER-G77, a ZodError occurring during the validation of AgenticOnboardingRunSchema in SentryApiService.updateAgenticOnboardingStatus.

The root cause was that the projectSlugs and issueIds fields in AgenticOnboardingRunSchema were defined using .nullable(), which only accepts null values. However, the backend API would sometimes omit these fields entirely when they had no value, resulting in them being undefined in the received data. Zod's .nullable() does not consider undefined valid, leading to validation failures.

The fix involves changing .nullable() to .nullish() for both projectSlugs and issueIds in packages/mcp-core/src/api-client/schema.ts. This modification allows the schema to correctly validate both null and undefined values for these fields, aligning with the backend's behavior. The existing .transform((v) => v ?? []) will continue to correctly default both null and undefined to an empty array.

Fixes MCP-SERVER-G77