feat(@inquirerer/test): add createTestFixture for CLI testing

[pyramation]

Summary

Adds a new createTestFixture() function to @inquirerer/test that provides a complete test harness for testing inquirerer-based CLI applications. This consolidates the duplicated TestFixture class pattern found in both packages/cli and pgpm/cli in the constructive repo.

The fixture handles:

• Creating temporary directories for test isolation
• Optionally copying fixture files from a source directory
• Setting up mock stdin/stdout streams via createTestEnvironment()
• Creating an Inquirerer prompter with the mock streams internally
• Running CLI commands with the test environment
• Cleanup of temporary directories

Usage:

import { createTestFixture } from '@inquirerer/test';
import { commands } from '../src/commands';

const fixture = createTestFixture({
  commands,
  fixtureRoot: FIXTURES_PATH,
  tmpPrefix: 'my-cli-test-',
  cliOptions: { version: '1.0.0' },
  argvTransform: (argv) => withDefaults(argv)  // optional
});

const { result, writeResults } = await fixture.runCmd({ _: ['init'] });
fixture.cleanup();

Review & Testing Checklist for Human

• No unit tests exist for the new fixture.ts - The test suite passes with --passWithNoTests. Consider whether tests should be added before merging, or if integration testing via constructive is sufficient.
• Verify the commands function signature matches constructive's CLIs - The expected signature is (argv: Partial<ParsedArgs>, prompter: Inquirerer, options: CLIOptions) => Promise<TResult>. Check that both packages/cli and pgpm/cli commands match this.
• Single environment per fixture - The TestEnvironment is created once when the fixture is instantiated. If runCmd is called multiple times, writeResults/transformResults will accumulate. Verify this is the intended behavior.

Test plan: After publishing this package, update constructive to use createTestFixture and verify existing tests still pass.

Notes

This is part of an ongoing effort to de-duplicate CLI test utilities between constructive's packages. A follow-up PR will update constructive to use this new fixture.

Link to Devin run: https://app.devin.ai/sessions/e5018850caff45ce8a126331548d69bc
Requested by: Dan Lynch (@pyramation)

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