fix(jfrog-oauth): fail when access_token is empty

Add a lifecycle precondition to the coder_script resource that validates
the JFrog access token is not empty. This prevents the module from
silently creating configurations with empty tokens when users haven't
authenticated via external auth.

Changes:
- Add precondition check on data.coder_external_auth.jfrog.access_token
- Replace main.test.ts with jfrog-oauth.tftest.hcl for proper data mocking
- Add comprehensive tests covering all package managers (npm, pypi, go,
  docker, conda, maven) with script content validation
- Tests verify actual generated configuration content, not just resource
  existence

Fixes #72

Author: matifali