fix: handle empty private and public key ctx structs

[strantalis]

Proposed Changes

Checklist

• I have added or updated unit tests
• I have added or updated integration tests (if appropriate)
• I have added or updated documentation

Testing Instructions

[c-r33d]

I have the following in my PR, which would be good to add to this PR. Would be added here:
https://github.com/opentdf/platform/blob/main/service/policy/db/key_access_server_registry.go#L403

var privateCtx []byte
	if r.GetPrivateKeyCtx() != nil {
		privateCtx, err = json.Marshal(r.GetPrivateKeyCtx())
		if err != nil {
			return nil, db.ErrMarshalValueFailed
		}
	}

[Copilot]

Pull Request Overview

This PR enhances key context handling by updating the unmarshalPrivatePublicKeyContext function and its usage across the codebase to better handle empty and nil key context structs. Key changes include:

• Refactoring unmarshalPrivatePublicKeyContext to return the key contexts instead of receiving them as parameters.
• Adding comprehensive tests in utils_test.go to cover various scenarios with empty and nil public/private key JSON.
• Updating key_access_server_registry.go to incorporate the refactored function and adjust private key context marshalling.

Reviewed Changes

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

File	Description
service/policy/db/utils_test.go	Added tests for handling empty and nil key contexts
service/policy/db/utils.go	Refactored unmarshalPrivatePublicKeyContext to return key contexts
service/policy/db/key_access_server_registry.go	Updated key creation and retrieval to use the new unmarshalPrivatePublicKeyContext

Comments suppressed due to low confidence (1)

service/policy/db/utils_test.go:201

• [nitpick] Duplicate test case name 'Nil public and private key pointers' appears twice, which might cause confusion during test reporting. Consider renaming one instance to clarify the distinct scenarios being tested.

name:    "Nil public and private key pointers",

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