…64376)

On double-checking the various Cody Gateway access management CRUD I
noticed that I did not implement the ability to remove the override
entirely properly - right now, removing a rate limit with `field_mask`
and no values results in a zero-value rate limit rather than a null one
as desired.

This change makes it so that if you provider a nil `*_rate_limit`
object, all field paths for that object is also set to nil, removing the
override and allowing the default active-license-plan-based rate limits
to apply.

This also adds a minor adjustment to Cody Gateway to correctly handle
zero durations for rate limit intervals, which should be treated as "no
access".

## Test plan

Unit/integration tests, and a manual test:

- Check out https://github.com/sourcegraph/sourcegraph/pull/64090 and
run `sg start dotcom`
- Go to a subscription, add a rate limit override 

![image](https://github.com/user-attachments/assets/9f417a60-5096-4f6e-8eea-1f0720cb4ada)
- Click the 🗑️ button to remove the rate limit override 

![image](https://github.com/user-attachments/assets/d53f4ae9-43d9-4bb2-a33b-7632f74ea2c8)
- Repeat above on each property