test(ci): add compile-only CI guard for broken test regression

[kangeunchan]

Enhancement Description

Prevent silent test breakages by adding a compile-only test gate in CI.

Summary

This PR adds a fast compile-only go test guard so test compilation regressions fail immediately in CI.

Previously, compile regressions could slip through:

• some CI paths emphasized runtime execution and could miss compile-only failures
• test package breakages were caught later than necessary

Now CI always validates that test code compiles even when runtime-heavy paths are skipped.

What Changed

1) Added compile-only test gate to CI

Updated:

• .github/workflows/ci.yml

Guard command:

• go test ./... -run='^$'

2) Kept runtime test behavior unchanged

• no runtime test semantics were altered
• this adds an early safety net before heavier jobs

3) Improved failure feedback timing

• compile errors are surfaced in the first CI stage
• reduces turnaround time for test-related fixes

4) Tests

Validation is CI-focused for this change.

Why This Is Needed

This is a CI reliability improvement.
Compile-time test failures should fail fast and predictably.

Behavior Notes / Regression Impact

No product runtime behavior change.
Only CI/test pipeline behavior is tightened.

Validation Performed

• go test ./... -run='^$'
• CI workflow run confirming compile-only gate execution