Fix Music3 contract spec validation

[0xShug0]

Summary

• remove invalid empty strip_prefix fields from the MiniMax Music3 package spec
• keep full spec/package validation strict for package/resource paths
• load schema-v1 runtime contracts without validating package-only metadata, so old bad embedded package metadata cannot break server option validation
• keep legacy/non-v1 specs returning no schema-v1 contract

Fixes #349.

Validation

• cmake --build build/debug --parallel \24 --target model_spec_system_test audiocpp_server audiocpp_cli
• build/debug/bin/model_spec_system_test
• pulled ghcr.io/0xshug0/audio.cpp:full-cpu and reproduced the current failure on POST /v1/models/load for MiniMax Music3: 500 with packages[0].strip_prefix: expected non-empty string
• reran the same prebuilt CPU image with only the fixed minimax_music3.json overlaid and the same load request returned 200 OK
• verified a legacy non-v1 OmniVoice inspect path still works