feat: add provider-aware model fallback to thread start

[celia-oai]

Why

Helper threads such as task title generation can request a model ID that is valid for the default OpenAI provider but unavailable from the active provider. With Amazon Bedrock, gpt-5.4-mini is rejected while the provider static catalog exposes Bedrock model IDs such as openai.gpt-5.5 and openai.gpt-5.4. This causes repeated background 404s and can surface a misleading turn error even when the main turn succeeds.

Clients need an explicit way to ask app-server to resolve an unavailable helper model to the active provider default. That fallback must remain limited to providers with an authoritative static catalog so custom or dynamically discovered model IDs are not rewritten based on an incomplete catalog.

Fixes #28741.

What changed

• Add the experimental allowProviderModelFallback option to thread/start, defaulting to false to preserve existing behavior.
• Thread the option through thread creation and model selection.
• When enabled for a static model manager, preserve requested models present in the catalog and replace unavailable models with the provider default.
• Continue preserving explicit model IDs for dynamic model managers without fetching a catalog solely to validate them.
• Document the new thread/start behavior in the app-server API overview.

Test

Temporary test-client harness:

ThreadStartParams {
    model: Some("gpt-5.4-mini".to_string()),
    allow_provider_model_fallback: true,
    ..Default::default()
}

Command:

CODEX_HOME=/tmp/codex-bedrock-thread-start-home \
CODEX_E2E_BEDROCK_THREAD_START_ONLY=1 \
./target/debug/codex-app-server-test-client \
  --codex-bin ./target/debug/codex \
  -c 'model_provider="amazon-bedrock"' \
  send-message-v2 --experimental-api ignored

Relevant output:

> "method": "thread/start",
> "params": {
>   "model": "gpt-5.4-mini",
>   "modelProvider": null,
>   "allowProviderModelFallback": true,
>   ...
> }

< "result": {
<   "model": "openai.gpt-5.5",
<   "modelProvider": "amazon-bedrock",
<   ...
< }