fix: improve error for unconfigured local providers (ollama/vllm) (#17328)

When a user sets `agents.defaults.model.primary: "ollama/gemma3:4b"`
but forgets to set OLLAMA_API_KEY, the error is a confusing
"unknown model: ollama/gemma3:4b". The Ollama provider requires any
dummy API key to register (the local server doesn't actually check it),
but this isn't obvious from the error.

Add `buildUnknownModelError()` that detects known local providers
(ollama, vllm) and appends an actionable hint with the env var name
and a link to the relevant docs page.

Before: Unknown model: ollama/gemma3:4b
After:  Unknown model: ollama/gemma3:4b. Ollama requires authentication
        to be registered as a provider. Set OLLAMA_API_KEY="ollama-local"
        (any value works) or run "openclaw configure".
        See: https://docs.openclaw.ai/providers/ollama

Closes #17328

src/agents/pi-embedded-runner/model.test.ts

@@ -340,4 +340,29 @@ describe("resolveModel", () => {
       },
     });
   });
+
+  it("includes auth hint for unknown ollama models (#17328)", () => {
+    // resetMockDiscoverModels() in beforeEach already sets find → null
+    const result = resolveModel("ollama", "gemma3:4b", "/tmp/agent");
+
+    expect(result.model).toBeUndefined();
+    expect(result.error).toContain("Unknown model: ollama/gemma3:4b");
+    expect(result.error).toContain("OLLAMA_API_KEY");
+    expect(result.error).toContain("docs.openclaw.ai/providers/ollama");
+  });
+
+  it("includes auth hint for unknown vllm models", () => {
+    const result = resolveModel("vllm", "llama-3-70b", "/tmp/agent");
+
+    expect(result.model).toBeUndefined();
+    expect(result.error).toContain("Unknown model: vllm/llama-3-70b");
+    expect(result.error).toContain("VLLM_API_KEY");
+  });
+
+  it("does not add auth hint for non-local providers", () => {
+    const result = resolveModel("google-antigravity", "some-model", "/tmp/agent");
+
+    expect(result.model).toBeUndefined();
+    expect(result.error).toBe("Unknown model: google-antigravity/some-model");
+  });
 });