[FEATURE]: v2 plugin API: expose credential list/activate on ctx.integration #51410

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

v2 only. I'm writing a v2 plugin that falls back when a subscription is rate limited or out of quota. The retry hook plus ctx.session.switchModel already handles switching models.

The missing piece is rotating accounts of the same provider. v2 already stores many credentials per integration (every login adds one and activates it), but plugins can only read them, not switch between them:

• ctx.integration.get() returns connections[]
• ctx.integration.connection.active() / resolve() exist
• there's no activate. It only exists on the HTTP client (client.credential.activate)

Proposed API

Core Credential.Service already implements this, so the change is only wiring in packages/plugin/src/promise/integration.ts + packages/core/src/plugin/host.ts:

export interface IntegrationDomain {
  // ...existing
  readonly credential: {
    readonly list: (integrationID: string) => Promise<Array<{ id: string; label: string }>>
    readonly activate: (credentialID: string) => Promise<void>
  }
}

Usage

await ctx.session.hook('retry', async (event) => {
  if (event.error.type !== 'provider.rate-limit' && event.error.type !== 'provider.quota') return
  const integrationID = event.model.providerID
  const active = await ctx.integration.connection.active(integrationID)
  const next = (await ctx.integration.credential.list(integrationID))
    .find((c) => c.id !== (active?.type === 'credential' ? active.id : undefined) && !isCoolingDown(c.id))
  if (!next) return
  await ctx.integration.credential.activate(next.id) // publishes credential.switched
  event.decision = { retry: true, delay: 0 }
})

Related: #38203 (recovering credentials after 401/403 would use the same primitive).