fix: use FQDN for CloudWatch agent auto-instrumentation endpoint (#392)

[pete911]

Closes #392.

The auto-instrumentation defaults injected by the operator's admission webhook pointed at the CloudWatch agent service using a short, 2-label hostname (cloudwatch-agent.amazon-cloudwatch) instead of a fully-qualified domain name. Because the value only
has one dot, resolving it depended entirely on the pod's DNS search list, which:

Changes

• pkg/instrumentation/helper.go: updated the cloudwatchAgentStandardEndpoint constant to the FQDN (cloudwatch-agent.amazon-cloudwatch.svc.cluster.local), matching the existing cloudwatchAgentWindowsEndpoint pattern.
• pkg/instrumentation/defaultinstrumentation.go: getDefaultInstrumentation() now references cloudwatchAgentStandardEndpoint/cloudwatchAgentWindowsEndpoint from helper.go instead of duplicating the hostname as a separate literal, so there's a single
  source of truth for both.
• pkg/instrumentation/defaultinstrumentation_test.go, helper_test.go, podmutator_test.go: updated hardcoded expected env var values to the new FQDN. TestContainsCloudWatchAgent's negative/edge-case assertions (path-embedded host, prefix-mismatch host,
  no-protocol-separator) were preserved with their original intent against the new FQDN boundary.
• README.md: updated the two example Instrumentation CR snippets to use the FQDN.

Testing

• go build ./... — pass
• go vet ./... — pass
• make test (full suite, envtest-backed) — all packages pass, including pkg/instrumentation
• Manually verified containsCloudWatchAgent() still correctly accepts/rejects each case in TestContainsCloudWatchAgent after the constant change (substring matching against the FQDN, including the windows-headless and negative cases)

Behavior change

None for existing clusters at default ndots — the FQDN resolves identically today. This unblocks operators who want to set ndots: 1 and removes the NXDOMAIN log noise from Route53 Resolver query logging.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.