Leave the state file untouched when an apply changes nothing - #152

TL;DR: An apply that changes nothing no longer rewrites the state file, which fixes the integration test failing on main since #148.

What & why

• main's integration test fails on Test_Apply_With_State_Subsequent_Run_No_Changes: applying the same group twice changed the state file's lastUpdated. Since Stop apply --enable-state deleting resources whose metadata changed #148, re-applying a managed resource replaces its entry with the latest metadata, which is intended, but it also bumped lastUpdated every time, so every run rewrote the state. The test only caught it when its two applies straddled a second, which is why Stop apply --enable-state deleting resources whose metadata changed #148 passed its own CI.
• Test_Apply_Gateway_Resources flakes one run in a hundred: the random interceptor fixture could draw priority 0, which Gateway rejects (priority must be greater than or equal to 1). It did on Plan creates with diffs, and apply the deletes of an emptied folder #149.

Changes

• AddManagedResource leaves the entry and lastUpdated alone when the stored entry is already identical; a real metadata change (labels, description) still updates both.
• Unit test pinning lastUpdated across an unchanged re-apply and a relabel.
• Fixture priority is 1 + rand.IntN(100).

Notes

• Resources are parsed YAML→JSON (ghodss/yaml), so their metadata has the same Go types as a state loaded from JSON and reflect.DeepEqual compares them reliably.
• go test ./... and go vet ./... pass locally; the integration suite needs a license, so this PR's CI run is its check.
• Plan creates with diffs, and apply the deletes of an emptied folder #149 needs rebasing onto this once merged.