fix: Correct GetRegionAvailability response type

[glennpratt]

The regions/{regionID}/availability API endpoint returns a JSON array of availability records (one per plan), not a single object.

https://techdocs.akamai.com/linode-api/reference/get-region-availability

✔️ How to Test

https://gist.github.com/glennpratt/e427a03cb749ccec3c3041c6e6396aea

go test -run TestGetRegionAvailability ./test/unit/...

[Copilot]

Pull request overview

This PR fixes the response type for the GetRegionAvailability function to correctly return an array of availability records (one per plan) instead of a single object, aligning with the actual Linode API behavior documented at https://techdocs.akamai.com/linode-api/reference/get-region-availability.

Changes:

• Updated GetRegionAvailability to return []RegionAvailability instead of *RegionAvailability
• Modified caching logic to properly handle array responses
• Updated test fixture and assertions to match the corrected array response format

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
regions_availability.go	Changed return type from pointer to single object to array, updated response handling and caching logic
test/unit/region_test.go	Updated test assertions to access array elements instead of direct object properties
test/unit/fixtures/region_availability_get.json	Changed fixture from single object to array containing availability records

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[srager13]

I can confirm that this is an issue. I get this error when calling GetRegionAvailability() in v1.61.0:
[002] json: cannot unmarshal array into Go value of type linodego.RegionAvailability

[glennpratt]

Awaiting #898