[Sanjays2402]

Fixes #980

UlimitsConfig.MarshalJSON writes a single limit as a bare number ("nproc": 65535), but there was no UnmarshalJSON to match, so unmarshalling a marshalled project with encoding/json failed:

json: cannot unmarshal number into Go struct field Project.services.app.ulimits.nproc of type types.UlimitsConfig

This adds UnmarshalJSON accepting both shapes MarshalJSON produces: a bare number for a single limit, and an object with soft and hard limits. A JSON null is left alone.

Repro, before the fix on main at 6ac9323: load a service with ulimits nproc 65535, json.Marshal the project, then json.Unmarshal it back. It failed with the error above; after the fix it round-trips, and the soft/hard form is unchanged.

Tests:

• New TestUlimitsJSONRoundTrip in loader/tests/ulimits_test.go, covering both the single-number and soft/hard forms through a full project JSON round-trip.
• go test ./... is green (types, loader, loader/tests and the rest of the tree).

[ndeloof]

we have tests in place to confirm marshalling / unmarshalling works as expected (see usage of roundTrip in loader/tests/), but unmarshalling relies on LoadWithContext, not on raw encoding/json.Unmarshal. This is the expected way: loader manages such multi-syntax support, and we have not plan to replicate the many corner cases with UnmarshalJSON functions on types