fix(iosxr): configure L2 and L3 MTU for different interface types

[sven-rosenzweig]

Due to IOS-XR MTU inheritance behavior for subinterfaces and bundle members, the following rules are sufficient for our use case:

• Physical interface: Configure L2 MTU and, if an IP address is present, configure L3 MTU.
• Bundle interface: Configure only the L2 MTU.
• Bundle member interface (physical): Do not configure MTU settings directly. L2 MTU is inherited from the bundle interface, and L3 MTU is configured on the corresponding subinterface.
• Subinterface (physical or bundle): Configure only the L3 MTU, using a default value of 1500 bytes.

In this commit, we assume a default L3 MTU of 1500 bytes. Cisco IOS-XR automatically adds:

• 4 bytes for 802.1Q encapsulation
• 8 bytes for 802.1ad (Q-in-Q) encapsulation

This allows subinterfaces to operate correctly with the default L3 MTU.

Official Cisco documentation:
https://www.cisco.com/c/de_de/support/docs/ios-nx-os-software/ios-xr-software/116350-trouble-ios-xr-mtu-00.html

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/ironcore-dev/network-operator/internal/provider/cisco/iosxr	41.20% (+1.28%)	👍

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/ironcore-dev/network-operator/internal/provider/cisco/iosxr/intf.go	64.71% (ø)	68	44	24
github.com/ironcore-dev/network-operator/internal/provider/cisco/iosxr/provider.go	29.11% (+1.78%)	158 (-3)	46 (+2)	112 (-5)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/ironcore-dev/network-operator/internal/provider/cisco/iosxr/intf_test.go
• github.com/ironcore-dev/network-operator/internal/provider/cisco/iosxr/provider_test.go