feat: let model providers own model discovery

[celia-oai]

Why

codex-models-manager had grown to own provider-specific concerns: constructing OpenAI-compatible /models requests, resolving provider auth, emitting request telemetry, and deciding how provider catalogs should be sourced. That made the manager harder to reuse for providers whose model catalog is not fetched from the OpenAI /models endpoint, such as Amazon Bedrock.

This change moves provider-specific model discovery behind provider-owned implementations, so the models manager can focus on refresh policy, cache behavior, picker ordering, and model metadata merging.

What Changed

• Introduced a ModelsManager trait with separate OpenAiModelsManager and StaticModelsManager implementations.
• Added ModelsEndpointClient so OpenAI-compatible HTTP fetching lives outside codex-models-manager.
• Moved /models request construction, provider auth resolution, timeout handling, and request telemetry into codex-model-provider via OpenAiModelsEndpoint.
• Added provider-owned models_manager(...) construction so configured OpenAI-compatible providers use OpenAiModelsManager, while static/catalog-backed providers can return StaticModelsManager.
• Added an Amazon Bedrock static model catalog for the GPT OSS Bedrock model IDs.
• Updated core/session/thread manager code and tests to depend on Arc<dyn ModelsManager>.
• Moved offline model test helpers into codex_models_manager::test_support.

Metadata References

The Bedrock catalog metadata is based on the official Amazon Bedrock OpenAI model documentation:

• Amazon Bedrock OpenAI models lists the Bedrock model IDs, text input/output modalities, and 128,000 token context window for gpt-oss-20b and gpt-oss-120b.
• Amazon Bedrock gpt-oss-120b model card lists the bedrock-runtime model ID openai.gpt-oss-120b-1:0, the bedrock-mantle model ID openai.gpt-oss-120b, text-only modalities, and 128K context window.
• OpenAI gpt-oss-120b model docs document configurable reasoning effort with low, medium, and high, plus text input/output modality.

The display names, default reasoning effort, and priority ordering are Codex-local catalog choices.

Test Plan

• Manually verified app-server model listing with an AWS profile:

CODEX_HOME="$(mktemp -d)" cargo run -p codex-app-server-test-client -- \
  --codex-bin ./target/debug/codex \
  -c 'model_provider="amazon-bedrock"' \
  -c 'model_providers.amazon-bedrock.aws.profile="codex-bedrock"' \
  -c 'model_providers.amazon-bedrock.aws.region="us-west-2"' \
  model-list

The response returned the Bedrock catalog with openai.gpt-oss-120b-1:0 as the default model and openai.gpt-oss-20b-1:0 as the second listed model, both text-only and supporting low/medium/high reasoning effort.

[celia-oai]

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[chatgpt-codex-connector]

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