fix: format string bug in GetPodMTU error logging

[dcoppa]

Changed error log in GetPodMTU to use podMTU (string input) instead of mtu (int result) when logging parse failures.

What type of PR is this?

bug

Which issue does this PR fix?:

This fixes the format string mismatch that causes:

{"level":"error","ts":"2025-10-02T23:03:58.442+0200","caller":"routed-eni-cni-plugin/cni.go:163","msg":"Failed to parse pod MTU %!s(int=0): strconv.Atoi: parsing \"\": invalid syntax"}

{"level":"error","ts":"2025-10-02T23:04:00.652+0200","caller":"networkutils/network_test.go:448","msg":"Failed to parse pod MTU %!s(int=0): strconv.Atoi: parsing \"abc\": invalid syntax"}

in the output of "make unit-test".

What does this PR do / Why do we need it?:

Bugfix

Testing done on this change:

Unit tests passed, the errors above are no longer displayed.

Will this PR introduce any new dependencies?:

No

Will this break upgrades or downgrades? Has updating a running cluster been tested?:

No / Yes

Does this change require updates to the CNI daemonset config files to work?:

No

Does this PR introduce any user-facing change?:

No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Copilot]

Pull Request Overview

This PR fixes a format string bug in error logging where the wrong variable was being used in the error message when parsing pod MTU fails.

• Fixed error log format string mismatch in GetPodMTU function
• Corrected variable usage from mtu (int) to podMTU (string) in error message

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