fix: Load env_files from top-level include during project interpolation

[MaheshThakur9152]

Summary
Fixes #13540
Fixes an issue where environment variables defined in a top-level include.env_file were not available for interpolation within the env_file of included services.

The Issue
When a top-level compose.yaml includes another file and provides an env_file for that include, those variables should be available for interpolation in the included services.

• Example: Top-level includes subproj with env_file: values.env (VAR=1). Sub-project service uses env_file with ${VAR?}.
• Before: Fails with required variable VAR is missing a value.
• After: Resolves correctly.

The Fix
I updated the CLI loader.go to explicitly pre-process top-level include directives:

1. Added collectIncludeEnvFiles to parse and resolve local env_file paths from includes.
2. Injected these files into the project load options via cli.WithEnvFiles.

Implementation Note
I implemented this as a pre-load step in the CLI rather than modifying the recursive loading logic in compose-go. This ensures the environment is correctly "primed" for interpolation with minimal risk of regression to the core engine.

Testing

• ✅ Unit Test: Added TestLoadProject_IncludeEnvFileInterpolation in loader_test.go to verify the fix deterministically.
• ✅ E2E: Added a regression fixture in pkg/e2e/fixtures/include-envfile-interpolation/ and verified with TestIncludeEnvFileInterpolation.

Verification
Validated locally with go test ./pkg/compose -run TestLoadProject_IncludeEnvFileInterpolation.

[ndeloof]

fix is wrong.
env file resolution for a sub-project used as include should take place in compose-go

[MaheshThakur9152]

fix is wrong. env file resolution for a sub-project used as include should take place in compose-go

Thanks for the review @ndeloof.
I suspected this might be the case. I implemented it in the CLI loader as a 'shim' to avoid modifying the recursive loading logic in compose-go initially.
I am happy to move this fix upstream. I will take a look at compose-spec/compose-go to implement the env file resolution within the include loader there. I'll close this once I have that PR ready.