runtime: Add max buffer size to inetstack from the network runtime

[iyzhang]

This PR fixes an issue where the inetstack was using the default implementation of max_buffer_size_bytes() rather than getting it from the underlying physical layer. For example, in Catnip, this meant that we were not using the configured DPDK max_body_size.

[Copilot]

Pull Request Overview

This PR adds proper propagation of max buffer size configuration from the underlying physical layer (layer1) up through all network stack layers. The change ensures that configured buffer sizes (such as DPDK's max_body_size in Catnip) are properly used instead of relying on default implementations.

• Implements max_buffer_size_bytes() method for DemiMemoryAllocator trait across all network layers
• Creates a chain of delegation from layer4 down to layer1 to retrieve the actual configured buffer size
• Fixes the issue where inetstack was not using the underlying physical layer's buffer size configuration

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/inetstack/mod.rs	Implements max_buffer_size_bytes() for SharedInetStack, delegating to layer4
src/inetstack/protocols/layer4/mod.rs	Implements max_buffer_size_bytes() for Peer, delegating to layer3
src/inetstack/protocols/layer3/mod.rs	Implements max_buffer_size_bytes() for SharedLayer3Endpoint, delegating to layer2
src/inetstack/protocols/layer2/mod.rs	Implements max_buffer_size_bytes() for SharedLayer2Endpoint, delegating to layer1

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[anandbonde]

Looks good!