hotfix: add GPT-6 provider models and defaults

[Yohjisakamoto]

Pull request type

• Provider compatibility

Outcome

Add GPT-6 Sol, Luna, and Astra to the Codex and OpenAI provider catalogs and routes. GPT-6 Sol becomes the default; GPT-5.6 models remain selectable and available as fallbacks.

Scope and compatibility

• Changed contracts, state, storage, protocol, CLI, GUI, TUI, or installation behavior: gateway and GUI defaults, bundled agent configurations, model catalog and routing, and provider request compatibility.
• Compatibility or migration risk: existing GPT-5.6 models and routes remain available.
• Explicitly out of scope: live provider calls and historical benchmark evidence.

Verification

cargo test -p provider --lib -> 179 passed
cargo test -p gateway --lib catalog_model_supported_by_provider_matches_registry_exactly -> passed
bun ./node_modules/typescript/bin/tsc --noEmit (apps/gui/app) -> passed
git diff --check -> passed

Repository-wide cargo fmt --all -- --check reports existing formatting differences in unrelated files. Live provider calls were not run.

Type-specific evidence

Provider: catalog, routing, and request payload behavior are covered by local tests; production availability still depends on the configured provider account.

Safety and responsibility

• No credentials, private session data, unsafe provider logs, or generated local state are included.
• Public evidence was sanitized according to the contribution guide.
• A human is the primary submitter and accepts responsibility for correctness, licensing, provenance, verification, and the statements in this pull request.
• I have the right to submit all included code, data, prompts, fixtures, and generated material under the repository license.

Meaningful AI assistance: Codex prepared the model catalog changes, local tests, and this PR description.