Add integration test to validate js-compute export/type resolution behavior

[harmony7]

The 3.39 change exposed legacy TypeScript configurations that relied on previous default export behavior. To prevent unintended changes to export or type resolution in the future, add a lightweight integration test to CI that validates how js-compute resolves under different TypeScript configurations.

This test should simulate real consumer behavior and fail if export or type resolution changes unexpectedly.

Scope

Create a minimal TypeScript consumer fixture and validate two scenarios:

1. Fastly configuration
   • tsconfig includes Fastly condition (current standard tooling behavior)
   • Expected: Fastly-specific types resolve successfully
2. Legacy/default configuration
   • No Fastly condition
   • Expected: Default types resolve according to current intended behavior (stub/default compatibility)

The CI job should run tsc and fail on unexpected resolution errors.

Acceptance Criteria

• A GitHub Actions job runs on PR and main.
• Two fixture projects exist:
  • consumer-fastly
  • consumer-default
• Both compile successfully under the current intended behavior.
• If export map or type resolution changes in a way that breaks either scenario, CI fails.

Notes

Goal is to detect accidental changes to:

• exports map
• conditional types resolution
• default vs fastly behavior