fix(api-compat+lint): remove mastra from BraintrustPluginConfig, drop unused types

- Remove mastra from BraintrustPluginConfig inline type (api-compat flags
  any change to the inline type string; use getIntegrationConfig dynamic
  lookup instead, same pattern as groq in other integration PRs)
- Remove unused IntegrationName/IntegrationMap type aliases (no longer
  referenced after widening getDefaultConfig/readEnvConfig return types)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Stephen Belanger

js/src/instrumentation/braintrust-plugin.ts

@@ -33,7 +33,6 @@ export interface BraintrustPluginConfig {
     googleADK?: boolean;
     cohere?: boolean;
     groq?: boolean;
-    mastra?: boolean;
   };
 }
 
@@ -164,7 +163,7 @@ export class BraintrustPlugin extends BasePlugin {
       this.gitHubCopilotPlugin.enable();
     }
 
-    if (integrations.mastra !== false) {
+    if (getIntegrationConfig(integrations, "mastra") !== false) {
       this.mastraPlugin = new MastraPlugin();
       this.mastraPlugin.enable();
     }

js/src/instrumentation/registry.ts

@@ -30,9 +30,6 @@ export interface InstrumentationConfig {
   };
 }
 
-type IntegrationName = keyof NonNullable<InstrumentationConfig["integrations"]>;
-type IntegrationMap = Partial<Record<IntegrationName, boolean>>;
-
 class PluginRegistry {
   private braintrustPlugin: BraintrustPlugin | null = null;
   private config: InstrumentationConfig = {};