feat: add native Scaleway Generative APIs provider

[claudecodebyia]

Summary

Adds first-class support for Scaleway Generative APIs as an OpenAI-compatible ForgeCode provider.

• registers scaleway as a built-in provider authenticated by SCW_SECRET_KEY
• supports both the default Scaleway endpoint and optional project-scoped URLs through SCW_PROJECT_ID
• exposes glm-5.2 with its 256k context window, reasoning, function calling, and parallel tool calling capabilities
• adds a Scaleway compatibility transformer that caps output at 16,384 tokens and normalizes reasoning effort to the values accepted by GLM-5.2
• adds an actionable IAM error when a key cannot access the configured project
• documents interactive login and legacy environment-variable configuration

Motivation

Scaleway follows the OpenAI Chat Completions contract, but two provider-specific details prevent a generic configuration from working reliably in ForgeCode:

1. project scoping is encoded in the URL and must remain optional because the default project uses the unscoped endpoint
2. ForgeCode's default output budget can exceed GLM-5.2's 16,384-token serverless limit, causing a 400 Bad Request

This integration handles both details centrally so users can select Scaleway like any other native provider.

User impact

Users can now run:

forge provider login scaleway
forge config set model scaleway glm-5.2

They may leave SCW_PROJECT_ID blank for the default project or provide a project ID when their IAM key has access to it.

Validation

• cargo test -p forge_domain provider --lib — 51 passed
• cargo test -p forge_app dto::openai::transformers --lib — 110 passed
• cargo test -p forge_repo provider::provider_repo --lib — 34 passed
• cargo test -p forge_repo test_enhance_error_scaleway_project_permissions --lib — passed
• cargo clippy -p forge_domain -p forge_app -p forge_repo --all-targets -- -D warnings — passed
• targeted rustfmt --check for every changed Rust file — passed
• live end-to-end ForgeCode request against Scaleway glm-5.2 using the default project endpoint — returned OK

References

• Scaleway Generative APIs authentication and project scoping
• Scaleway supported models

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