Add ContextArgs type export and update default context file template

Agent-Logs-Url: https://github.com/counterfact/api-simulator/sessions/e9e5f399-6483-4bfb-8c4e-1df903f65bfb

Author: Copilot

.changeset/strongly-typed-load-context.md

@@ -0,0 +1,5 @@
+---
+"counterfact": minor
+---
+
+Export `ContextArgs` type from generated `types/scenario-context.ts` so that `_.context.ts` files can strongly type the `loadContext` and `readJson` parameters received in the Context constructor. The default `_.context.ts` template now imports and uses `ContextArgs`.

src/typescript-generator/generate.ts

@@ -256,6 +256,11 @@ function buildApplyContextContent(contextFiles: ContextFileInfo[]): string {
     "/** A scenario function that receives the live REPL environment */",
     "export type Scenario = ($: ApplyContext) => Promise<void> | void;",
     "",
+    "/** Arguments passed to the Context class constructor in _.context.ts files */",
+    'export type ContextArgs = Pick<ApplyContext, "loadContext"> & {',
+    "  readJson: (relativePath: string) => Promise<unknown>;",
+    "};",
+    "",
   ];
 
   return parts.join("\n");

src/typescript-generator/repository.ts

@@ -148,7 +148,9 @@ export class Repository {
 
     await fs.writeFile(
       contextFilePath,
-      `/**
+      `import type { ContextArgs } from "../types/scenario-context.js";
+
+/**
 * This is the default context for Counterfact.
 *
 * It defines the context object in the REPL 
@@ -159,7 +161,7 @@ export class Repository {
 * See https://counterfact.dev/docs/usage.html#working-with-state-the-codecontextcode-object-and-codecontexttscode
 */
 export class Context {
-
+  constructor(_args: ContextArgs) {}
 }
 `,
     );

test/typescript-generator/generate.test.ts

@@ -80,6 +80,12 @@ describe("scenario-context type generation", () => {
       expect(content).toContain(
         "export type Scenario = ($: ApplyContext) => Promise<void> | void;",
       );
+      expect(content).toContain(
+        'export type ContextArgs = Pick<ApplyContext, "loadContext"> & {',
+      );
+      expect(content).toContain(
+        "  readJson: (relativePath: string) => Promise<unknown>;",
+      );
     });
   });