Preserve configured model options missing from preset catalog

[garrza]

Summary

• Preserve raw configured or selected model IDs when the active preset catalog does not have metadata for them
• Keep known hidden presets hidden unless they are the current model
• Use a single preset snapshot for legacy model list state

Fixes #236

Testing

• cargo test
• cargo clippy --all-targets -- -D warnings

[garrza]

TLDR

The bug was this:

1. The session starts with config.model = "gpt-5.5".
2. ACP asks the model catalog for presets.
3. If the catalog does not contain a preset for gpt-5.5, ACP still shows it because it is the current model.
4. User switch to an older model.
5. Now gpt-5.5 is no longer current.
6. The old picker code only showed:
   • models with show_in_picker = true
   • plus the current model
7. So gpt-5.5 disappeared.

The PR adds a small session memory:

  remembered_raw_models: Vec<String>

It is seeded from the configured model when the thread starts. So if the thread starts with gpt-5.5, ACP remembers that raw model ID.

The core fix is the picker logic so Codex ACP does not lose a valid configured model just because that model is missing from the current UI preset catalog. The bug happened when source 1 had gpt-5.5, but source 2 did not.

Important guardrail: if a model does exist in the preset catalog but has show_in_picker = false, we still hide it after switching away. So the PR does not bypass intentional hidden model behavior.