smartcontract: enforce 9000-byte MTU on links and device interfaces

[elitegreg]

Summary

Change the enforced MTU for links and non-CYOA/non-DIA device interfaces from 2048 to 9000 bytes. CYOA/DIA interfaces remain at 1500. Add onchain validation using the existing InvalidMtu error code, and make the CLI --mtu flag optional with correct defaults.

Motivation

The network should use jumbo frames (9000 bytes) by default for all fabric-facing links and interfaces. The previous 2048-byte MTU was a placeholder that does not reflect the intended production configuration. Enforcing this at both the CLI and onchain level prevents misconfigured links and interfaces from being registered.

Changes

Onchain (smartcontract program)

• Validate MTU in link create/update processors: must be 9000, else return InvalidMtu (error 46)
• Validate MTU in device interface create/update processors: must be 9000 for non-CYOA/non-DIA interfaces, 1500 for CYOA/DIA interfaces

CLI

• doublezero link create wan/dzx: --mtu becomes optional, defaults to 9000, rejects other values
• doublezero link update: --mtu validates 9000 if provided
• doublezero device interface create: defaults to 9000 (non-CYOA/DIA) or 1500 (CYOA/DIA), rejects other values
• doublezero device interface update: same validation rules
• Remove validate_parse_mtu range validator (no longer meaningful)

Tests

• Update all test fixtures from 2048/1500 to 9000 for non-CYOA interfaces and links
• Add onchain tests for InvalidMtu rejection
• Update test script (start-test.sh) MTU values