[test] Add comprehensive upgrade workflow tests to 80%+ coverage

[kangeunchan]

Enhancement Description

Add unit and workflow tests for upgrade logic to reach 80%+ coverage and reduce risk in the highest-impact path.

Background

Upgrade workflow contains critical operations (propose/vote/switch/execute/resume) with 0% test coverage, increasing regression risk and making refactoring unsafe.

Scope

Introduce mocks/fakes for upgrade dependencies

• Files: internal/application/upgrade/** (tests)
• Files: internal/application/ports/** (mock interfaces as needed)

Add test suites

• Happy path workflows (10)
• Error injection tests (15): RPC failures, timeouts, state corruption
• State transition tests (10): valid/invalid transitions
• Resume tests (12): resume from each stage

Target packages

• Files: internal/application/upgrade/**

Non-Goals

• Refactoring upgrade logic solely to improve testability (keep changes minimal)
• Expanding test coverage for unrelated packages in same PR
• End-to-end network integration tests that require real chains (unless already available)

Risks and Open Questions

• Mock fidelity: tests must reflect real sequencing and failure modes
• Resume/state persistence logic must be validated carefully to avoid false confidence
• Tests must remain stable and not rely on timing-sensitive behavior

Validation Plan

Unit and Integration Checks

• go test ./... with focus on internal/application/upgrade
• Coverage check for upgrade package (target 80%+)
• Deterministic unit tests with controlled fakes (no sleeps)

End-to-End Checks

• If CI supports: run a lightweight upgrade smoke in a sandbox environment
• Validate resume from a persisted state artifact (fixture-based)

Evidence Required in Issue Updates

• Coverage report output for internal/application/upgrade
• Logs or assertions demonstrating resume from each stage
• Failure injection evidence (expected errors and preserved state)

Acceptance Criteria

• internal/application/upgrade coverage ≥ 80%
• 50+ test cases across the categories
• All state transitions and resume stages covered
• Tests run deterministically and in parallel where possible

Deliverables

• PR adding upgrade mocks/fakes and test suites
• Documentation notes describing test strategy and fixtures

[kangeunchan]

pull request: #126