test: isolate passive chatd internal tests (#26369)

Make `newInternalTestServer` use option functions for logger, clock, and
worker startup, and make it passive by default so internal chatd tests
only opt into background execution when they need a real worker.

Use the passive server path in `TestAwaitSubagentCompletion` for the
state-driven subtests, keep `ContextCanceled` explicitly active for real
provider cancellation coverage, and keep the fail-fast default AI
provider base URL so accidental provider calls still fail immediately.

Closes CODAGT-586
Closes https://github.com/coder/internal/issues/1549

coderd/database/dbgen/dbgen.go

@@ -218,12 +218,13 @@ func AIProvider(t testing.TB, db database.Store, seed database.AIProvider, munge
 		displayName = sql.NullString{String: name, Valid: true}
 	}
 	params := database.InsertAIProviderParams{
-		ID:            id,
-		Type:          provType,
-		Name:          name,
-		DisplayName:   displayName,
-		Enabled:       takeFirst(seed.Enabled, true),
-		BaseUrl:       takeFirst(seed.BaseUrl, "https://api.example.com/"),
+		ID:          id,
+		Type:        provType,
+		Name:        name,
+		DisplayName: displayName,
+		Enabled:     takeFirst(seed.Enabled, true),
+		// Use an unsupported scheme so leaked test provider calls fail immediately without retries.
+		BaseUrl:       takeFirst(seed.BaseUrl, "invalid://test.invalid/"),
 		Settings:      seed.Settings,
 		SettingsKeyID: seed.SettingsKeyID,
 	}