Fix raw TOML agent spawn model/provider override behavior

[loydccc]

Summary

Fixes model override behavior during agent spawn so explicit manifest model selections are respected.

When spawning agents, default_model is now only overlaid if the manifest model config is still at built-in defaults (and has no per-agent �pi_key_env/�ase_url override).

Problem

Raw TOML agent manifests could specify a custom provider/model, but spawn logic still replaced them with config.default_model whenever �pi_key_env and �ase_url were absent.

Changes

• Added should_apply_default_model_overlay(model: &ModelConfig) -> bool in crates/openfang-kernel/src/kernel.rs.
• Updated both spawn-time and restore-time default model overlay checks to use this helper.
• Added regression tests:

  • est_should_apply_default_model_overlay_for_built_in_defaults
    

  • est_should_not_apply_default_model_overlay_for_explicit_model
    

  • est_should_not_apply_default_model_overlay_with_custom_key_or_url
    

Validation

• cargo fmt --all
• cargo test -p openfang-kernel default_model_overlay

Closes #189