Issue for this PR

Closes #28524

Type of change

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

What does this PR do?

Declaring a second Vertex AI provider entry under a custom ID (say google-vertex-eu for the eu multi-region) silently skips the Vertex loader today. The resulting SDK has no ADC credentials and requests fail with Model not found. The custom-loader iteration in packages/opencode/src/provider/provider.ts walks Object.entries(custom(dep)), keyed by exact provider ID, so anything that isn't a canonical loader name is dropped.

This PR walks the merged provider database instead. Each entry resolves to a loader by exact-ID match (canonical behaviour, untouched) or by unique-npm match. When the npm is shared (the @ai-sdk/openai-compatible family of seven loaders, plus the @ai-sdk/azure pair), the resolver returns undefined and the entry falls through to the generic SDK path. That keeps the change narrow. Explicit aliasing inside those shared-npm groups is a separate problem; #28489 attacks one slice of it with an auth_provider config field.

I also moved the literal model.providerID === "X" discriminations in provider.ts, transform.ts, and agent.ts to model.api.npm === NPM.X wherever the canonical loader uniquely owns its npm. Sites inside the openai-compatible family stay ID-keyed; npm can't discriminate among them. A new packages/opencode/src/provider/npm.ts holds the npm constants so a typo at a call site is a compile error.

Drive-by: the google-vertex-anthropic loader was reading project and location from env vars only and silently ignoring provider.options. Fixed to mirror google-vertex (config first, env fallback). Regression test included.

How did you verify your code works?

Seven new tests in packages/opencode/test/provider/provider.test.ts cover the alias path, canonical-entry regression, ambiguous-npm fallthrough for the azure pair and the openai-compatible family, the google-vertex-anthropic config gap, and the migrated Bedrock cross-region small-model picker.

bun test test/provider/ test/agent/   # 390 pass, 0 fail
bun run typecheck                     # clean

Manual, with the config below in opencode.json and Google ADC set up (gcloud auth application-default login):

opencode run --model google-vertex/gemini-2.5-pro --log-level DEBUG "ping"
opencode run --model google-vertex-eu/gemini-3.5-flash --log-level DEBUG "ping"

Both succeed. Debug logs show each request hitting its configured regional endpoint with an Authorization: Bearer … header from ADC.

Example

{
  "provider": {
    "google-vertex": {
      "options": { "project": "P", "location": "europe-west4" },
      "models": { "gemini-2.5-pro": {} }
    },
    "google-vertex-eu": {
      "npm": "@ai-sdk/google-vertex",
      "options": { "project": "P", "location": "eu" },
      "models": { "gemini-3.5-flash": {} }
    }
  }
}

Screenshots / recordings

N/A — backend/provider logic change.

Checklist

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