ensure custom EndpointResolver is still used when either EKS_POD_EXECUTION_ROLE environment variable is set OR when role_arn parameter is set.

[awprice]

Description of changes

Ensure custom EndpointResolver is still used when either EKS_POD_EXECUTION_ROLE environment variable is set OR when role_arn parameter is set.

This ensures the endpoint parameter is still used in either of these cases.

Testing

I have confirmed with the newly added unit test that this bug is fixed:

Before:

$ go test ./kinesis/...
time="2025-09-18T17:21:09+10:00" level=warning msg="[kinesis 0] Found record with 6291478 bytes, truncating to 1MB, stream=stream\n"
time="2025-09-18T17:21:09+10:00" level=warning msg="[kinesis 0] Found record with 6291477 bytes, truncating to 1MB, stream=stream\n"
time="2025-09-18T17:21:09+10:00" level=error msg="[kinesis 0] The partition key could not be found in the record, using a random string instead"
time="2025-09-18T17:21:09+10:00" level=error msg="[kinesis 0] The partition key could not be found in the record, using a random string instead"
--- FAIL: TestNewPutRecordsClient_CustomEndpointWithRoles (0.00s)
    --- FAIL: TestNewPutRecordsClient_CustomEndpointWithRoles/with_role_arn (0.00s)
        kinesis_test.go:453: 
            	Error Trace:	/Users/<redacted>/go/src/github.com/aws/amazon-kinesis-streams-for-fluent-bit/kinesis/kinesis_test.go:453
            	Error:      	Not equal: 
            	            	expected: "https://kinesis.custom-domain.local"
            	            	actual  : "https://kinesis.us-west-2.amazonaws.com"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-https://kinesis.custom-domain.local
            	            	+https://kinesis.us-west-2.amazonaws.com
            	Test:       	TestNewPutRecordsClient_CustomEndpointWithRoles/with_role_arn
            	Messages:   	Custom endpoint should be preserved when using role-based authentication
    --- FAIL: TestNewPutRecordsClient_CustomEndpointWithRoles/with_eks_role (0.00s)
        kinesis_test.go:453: 
            	Error Trace:	/Users/<redacted>/go/src/github.com/aws/amazon-kinesis-streams-for-fluent-bit/kinesis/kinesis_test.go:453
            	Error:      	Not equal: 
            	            	expected: "https://kinesis.custom-domain.local"
            	            	actual  : "https://kinesis.us-west-2.amazonaws.com"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-https://kinesis.custom-domain.local
            	            	+https://kinesis.us-west-2.amazonaws.com
            	Test:       	TestNewPutRecordsClient_CustomEndpointWithRoles/with_eks_role
            	Messages:   	Custom endpoint should be preserved when using role-based authentication
    --- FAIL: TestNewPutRecordsClient_CustomEndpointWithRoles/with_both_roles (0.00s)
        kinesis_test.go:453: 
            	Error Trace:	/Users/<redacted>/go/src/github.com/aws/amazon-kinesis-streams-for-fluent-bit/kinesis/kinesis_test.go:453
            	Error:      	Not equal: 
            	            	expected: "https://kinesis.custom-domain.local"
            	            	actual  : "https://kinesis.us-west-2.amazonaws.com"
            	            	
            	            	Diff:
            	            	--- Expected
            	            	+++ Actual
            	            	@@ -1 +1 @@
            	            	-https://kinesis.custom-domain.local
            	            	+https://kinesis.us-west-2.amazonaws.com
            	Test:       	TestNewPutRecordsClient_CustomEndpointWithRoles/with_both_roles
            	Messages:   	Custom endpoint should be preserved when using role-based authentication
FAIL
FAIL	github.com/aws/amazon-kinesis-streams-for-fluent-bit/kinesis	0.991s
?   	github.com/aws/amazon-kinesis-streams-for-fluent-bit/kinesis/mock_kinesis	[no test files]
FAIL

After:

$ go test ./kinesis/...
ok  	github.com/aws/amazon-kinesis-streams-for-fluent-bit/kinesis	0.968s
?   	github.com/aws/amazon-kinesis-streams-for-fluent-bit/kinesis/mock_kinesis	[no test files]