fix(provider): preserve config precedence after model hooks

[shlroland]

Issue for this PR

Closes #25630

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

After PR #25167, the plugin provider.models() hook runs before config providers are merged into the providers map. This means the hook looks up database[providerID] which only contains providers from the models.dev catalog. Custom providers declared in opencode.json that aren't in the catalog are skipped entirely.

This PR fixes the regression by:

1. Extracting config application into a reusable applyConfig() function — the inline config-to-database merge is now a function that can operate on any Record<string, Info> target.

2. Running plugin model hooks on providers after env/auth/custom loaders — this ensures custom providers from user config are present in the providers map when hooks execute.

3. Calling applyConfig(providers) after hooks — this restores the intended precedence: user config overrides hook-supplied models, while still allowing hooks to add new models to custom providers.

How did you verify your code works?

• Added a test "provider hook runs for config-only providers and preserves config model overrides" that:

  • Creates a custom provider "demo" in config with a "chat" model named "Config Chat"
  • Creates a plugin hook that adds a "live" model and renames "chat" to "Hook Chat"
  • Verifies the "live" model from the hook exists (hook can add models to custom providers)
  • Verifies "chat" model name is "Config Chat" (config takes precedence over hook)

• All existing provider tests pass

• Typecheck passes

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR