fix: show helpful error when docker is missing during bundling fallback

[knQzx]

Summary

• when esbuild is not installed and cdk falls back to docker bundling, a missing docker would show a cryptic spawnSync docker ENOENT error
• now it clearly says docker is not installed and suggests installing esbuild locally as an alternative
• added a test for the ENOENT case in dockerExec

Test plan

• added unit test that stubs spawnSync to return ENOENT and checks for the new error message
• verified the error includes both the "not installed" explanation and the esbuild suggestion

fixes #22997

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

✅ A exemption request has been requested. Please wait for a maintainer's review.

[knQzx]

Exemption Request

this change only improves an error message when docker is not found (ENOENT) during asset bundling - it doesn't change any cloudformation output or runtime behavior. an integration test for this would require docker to not be installed, which doesn't make sense in a normal test environment.

the fix is fully covered by unit tests in packages/aws-cdk-lib/core/test/private/asset-staging.test.ts that mock spawnSync to return an ENOENT error and verify the improved error message is thrown.

requesting pr-linter/exempt-integ-test label.