test(upgrade): add comprehensive workflow coverage with deterministic fakes

[kangeunchan]

Enhancement Description

Add comprehensive upgrade workflow tests to reach 80%+ package coverage and reduce regression risk.

Summary

This PR expands deterministic tests for internal/application/upgrade across state transitions, resume paths, and RPC-driven flows.

Previously, upgrade coverage was insufficient for high-confidence refactors:

• critical branches were under-tested
• resumable/gov workflow paths had limited direct verification
• behavior changes were harder to review safely

Now the package has broad unit/workflow coverage with reusable fakes.

What Changed

1) Added reusable test scaffolding and fake RPC harness

Added:

• internal/application/upgrade/test_helpers_test.go
• internal/application/upgrade/eth_rpc_test.go

2) Added state and resume behavior suites

Added:

• internal/application/upgrade/state_transitioner_test.go
• internal/application/upgrade/state_detector_test.go
• internal/application/upgrade/resume_test.go
• internal/application/upgrade/switch_test.go

3) Added execution/workflow coverage

Added:

• internal/application/upgrade/propose_vote_test.go
• internal/application/upgrade/execute_test.go
• internal/application/upgrade/resumable_execute_test.go

4) Tests

Coverage target met for package:

• internal/application/upgrade coverage: 80.3%

Why This Is Needed

This is a reliability and testability improvement for a critical workflow.
Higher deterministic coverage reduces refactor risk and improves review confidence.

Behavior Notes / Regression Impact

No intentional runtime behavior change.
Changes are test-only.

Validation Performed

• go test ./internal/application/upgrade -count=1
• go test ./internal/application/upgrade -coverprofile=/tmp/upgrade.cover -count=1
• go tool cover -func=/tmp/upgrade.cover
• go test ./...