fix: add Vivgrid provider

[fanweixiao]

Summary

Add Vivgrid as a built-in LLM provider using the OpenAI Responses API, including Vivgrid-hosted GPT-5.4 and Codex model entries.

Context

Vivgrid documents Codex-style integrations using https://api.vivgrid.com/v1 with VIVGRID_KEY and the Responses wire API. This project already supports multiple built-in providers through provider registry metadata, so Vivgrid can be integrated by registering it as a built-in provider and routing it through the existing OpenAI Responses backend.

Changes

• Added ProviderId::VIVGRID as a built-in provider
• Added Vivgrid provider registry config with VIVGRID_KEY
• Configured Vivgrid to use OpenAIResponses
• Added Vivgrid model entries for gpt-5.4, gpt-5.2-codex, and gpt-5.1-codex
• Extended provider tests to cover Vivgrid ID parsing, display name, built-in registration, and provider config loading

Key Implementation Details

Vivgrid is registered as a built-in provider in the domain layer so it participates in provider ordering, display-name rendering, and string parsing consistently with the other first-party providers. In the provider registry, Vivgrid is configured to hit https://api.vivgrid.com/v1/responses and use VIVGRID_KEY, matching Vivgrid’s published integration guidance. The provider is wired to the existing OpenAIResponses response type so no new repository implementation was required.

Use Cases

• Configure Forge to use Vivgrid without relying on generic custom-provider setup
• Run coding workflows against Vivgrid-hosted Codex models
• Use Vivgrid-hosted GPT-5.4 as a built-in selectable model provider

Testing

cargo test -p forge_domain test_vivgrid_from_str

cargo test -p forge_repo provider_repo::tests::test_load_provider_configs is currently blocked in this environment because protoc is not installed.

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[tusharmath]

We need a few test keys to verify if this works.

[fanweixiao]

We need a few test keys to verify if this works.

No problem, you can register an account on vivgrid, and send your account to me (cc@vivgrid.com), I'll sponsor credits to you.

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[fanweixiao]

Hi @tusharmath, I've already tested. See if this PR can be merged?

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[amitksingh1490]

@fanweixiao can you please rebase this once