refactor(session): replace MigrationEngine and isolate migrate command (#1195)

* refactor(session): simplify migration preview

Refs #1140

* refactor(session): split migration command

Keep session command modules below the production file-size limit without changing CLI behavior.

Author: ousamabenyounes

src/__tests__/migrate-guard.test.ts

@@ -11,7 +11,7 @@ import { describe, expect, it, beforeAll, afterAll } from 'vitest';
 import os from 'node:os';
 import type { AgentAdapter, SessionMeta } from '../session-flow/adapters/base.js';
 import { ADAPTER_REGISTRY, type AdapterFactory } from '../session-flow/adapters/index.js';
-import { fidelityFromSession } from '../session-flow/migrate.js';
+import { fidelityFromSession, previewMigration } from '../session-flow/migrate.js';
 import { AgentSession } from '../session-flow/agent-session.js';
 import type { Session } from '../session-flow/ir.js';
 
@@ -217,3 +217,21 @@ describe('fidelityFromSession — 真实统计', () => {
     });
   });
 });
+
+describe('previewMigration', () => {
+  it('returns the source, target, session metadata, and computed fidelity', () => {
+    const session = makeSession();
+    const sourcePlatform = session.platform;
+    const targetPlatform = 'cursor';
+
+    expect(previewMigration(session, sourcePlatform, targetPlatform)).toEqual({
+      sourcePlatform,
+      targetPlatform,
+      sessionId: session.sessionId,
+      sessionTitle: session.title,
+      cwd: session.cwd,
+      messageCount: session.messages.length,
+      fidelity: fidelityFromSession(session, targetPlatform),
+    });
+  });
+});

src/__tests__/session-cmd.test.ts

@@ -32,6 +32,7 @@ const mocks = vi.hoisted(() => ({
   adaptersByPlatform: {} as Record<string, unknown>,
   /** migrate.js mock 的 preview 返回值（Error 时抛出）；agent-session.js mock 的 copy 返回值 */
   previewResult: null as unknown,
+  previewCalls: [] as Array<{ session: unknown; source: string; target: string }>,
   copyResult: null as unknown,
   /** The OS the commands' `noAdapter` checks judge by; this machine's when unset. */
   os: undefined as NodeJS.Platform | undefined,
@@ -119,14 +120,9 @@ vi.mock('../session-flow/adapters/index.js', () => ({
 }));
 
 vi.mock('../session-flow/migrate.js', () => ({
-  MigrationEngine: class {
-    constructor(
-      public readonly source: string,
-      public readonly target: string,
-    ) {}
-    preview(): unknown {
-      return mocks.previewResult;
-    }
+  previewMigration: (session: unknown, source: string, target: string): unknown => {
+    mocks.previewCalls.push({ session, source, target });
+    return mocks.previewResult;
   },
   resolveTargetCwd: (explicit: string | undefined, sourceCwd: string | undefined) => explicit ?? sourceCwd,
 }));
@@ -186,6 +182,7 @@ beforeEach(() => {
   mocks.gitCalls.length = 0;
   mocks.adaptersByPlatform = {};
   mocks.previewResult = null;
+  mocks.previewCalls.length = 0;
   mocks.copyResult = null;
   mocks.os = undefined;
   mocks.heldArchiveLock = '';
@@ -714,16 +711,13 @@ describe('session migrate interactive picker', () => {
       filePath: '/tmp/tgt.jsonl',
       fidelity: { score: 1 },
     };
-    const { MigrationEngine } = await import('../session-flow/migrate.js');
-    const previewSpy = vi.spyOn(MigrationEngine.prototype, 'preview');
-
     feedAnswers(['n', '11']);
     await runSession('migrate', '-s', 'fakeplat', '-t', 'tgtplat', '--cwd', '/run/dir');
 
     // The picker is on page 2 by the time the number is entered, and the
     // selected id is the 11th newest -- unreachable before paging existed.
     expect(out.join('\n')).toContain('page 2/2');
-    expect(previewSpy).toHaveBeenCalledWith(sessions[1]);
+    expect(mocks.previewCalls).toContainEqual({ session: sessions[1], source: 'fakeplat', target: 'tgtplat' });
   });
 
   it('cancels on an empty answer instead of paging forever', async () => {

src/session-flow/migrate.ts

@@ -258,30 +258,21 @@ export function resolveTargetCwd(
   return fallback;
 }
 
-// ---------------------------------------------------------------------------
-// MigrationEngine
-// ---------------------------------------------------------------------------
-
-export class MigrationEngine {
-  constructor(
-    private sourcePlatform: string,
-    private targetPlatform: string,
-  ) {}
-
-  /** What migrating `session`, read from the source platform, would carry over. */
-  preview(session: Session): MigrationPreview {
-    const fidelity = fidelityFromSession(session, this.targetPlatform);
-
-    return {
-      sourcePlatform: this.sourcePlatform,
-      targetPlatform: this.targetPlatform,
-      sessionId: session.sessionId,
-      sessionTitle: session.title,
-      cwd: session.cwd,
-      messageCount: session.messages.length,
-      fidelity,
-    };
-  }
+/** What migrating `session`, read from `sourcePlatform`, would carry over. */
+export function previewMigration(
+  session: Session,
+  sourcePlatform: string,
+  targetPlatform: string,
+): MigrationPreview {
+  return {
+    sourcePlatform,
+    targetPlatform,
+    sessionId: session.sessionId,
+    sessionTitle: session.title,
+    cwd: session.cwd,
+    messageCount: session.messages.length,
+    fidelity: fidelityFromSession(session, targetPlatform),
+  };
 }
 
 export { getAdapter, listAvailablePlatforms, listInstalledPlatforms };