Mock everything and your suite is fast, stable and worthless: it tests your mocks. Mock nothing and it is slow, flaky and dependent on every third party your app touches. The senior contribution is the policy between those, and it needs to be written down rather than left to per-test instinct.
Three tiers. Every test belongs to exactly one.
Tier one: fully real. Fifteen to thirty tests covering the critical journeys. Real server, real database, no interception at all. These are your contract check. When the backend changes shape, these break, which is the entire point.