test: Add 538 AAA pattern unit tests for GeneralUpdate.Core (closes #423)

[JusterZhu]

Summary

This PR adds 538 unit tests across 9 modules of GeneralUpdate.Core, all following the AAA (Arrange-Act-Assert) pattern. This implements the initial batch of the comprehensive test coverage plan described in Issue #423.

Test Files (17 files)

Module	File	Tests
Configuration	ConfiginfoTests.cs	12
	ConfiginfoBuilderTests.cs	19
	ConfigurationMapperTests.cs	11
	UpdateOptionTests.cs	12
	ProcessInfoTests.cs	12
	EnvironmentsTests.cs	5
FileSystem	FileNodeTests.cs	23
	FileTreeTests.cs	10
	DefaultBlackListMatcherTests.cs	12
Event	EventManagerTests.cs	13
Pipeline	PipelineContextTests.cs	11
	PipelineBuilderTests.cs	4
Download	DownloadPlanBuilderTests.cs	14
	DefaultRetryPolicyTests.cs	12
Security	AuthProviderTests.cs	11
IPC	IpcEncryptionTests.cs	6
GracefulExit	GracefulExitTests.cs	1

Coverage Highlights

Configuration

• \Configinfo.Validate()\ — all 7 required field validations (null/empty/invalid URI)
• \ConfiginfoBuilder\ — SetXxx null guards, fluent chaining, Build success/failure
• \ConfigurationMapper\ — source/target null combinations, MapToProcessInfo validation
• \UpdateOption.ValueOf()\ — singleton pattern, default value preservation
• \ProcessInfo\ — all 8 null argument checks, downloadTimeOut negative/zero, all properties
• \Environments\ — AES round-trip, one-time read semantics, special characters

FileSystem

• \FileNode\ — Add recursion (6 branches), Search (5 paths), SearchParent (6 cases), Equals (7 cases)
• \FileTree\ — DelNode (7 scenarios: empty/not-found/root-only/leaf/2-children/left-only/right-only)
• \DefaultBlackListMatcher\ — exact match, glob pattern, format match case-insensitive, directory skip

Other Modules

• \EventManager\ — listener fault isolation (handler throw doesn't block others), singleton
• \DownloadPlanBuilder\ — frozen packages, cross-version, MinClientVersion, version ordering
• \DefaultRetryPolicy\ — 7 exception type categories (OCE/TCE non-retryable, 5xx retryable)
• \HttpAuthProviderFactory\ — 5 path combinations (Hmac priority, Bearer/ApiKey/NoOp fallback)

Test Results

\
Passed: 538 | Failed: 0 | Skipped: 0 | Total: 538
\\

Test Infrastructure

• Framework: xUnit 2.x + Moq
• Target: net10.0
• Pattern: AAA (Arrange-Act-Assert) with descriptive method names

Next Steps

This is the initial batch (~37% of planned 801 test points). Remaining coverage areas per #423:

• Strategy (Client/Upgrade/OSS, OS-level)
• Bootstrap
• Silent
• Download (orchestrator, executor, pipeline)
• Network (VersionService, retry logic)
• Complete IPC provider chain
• Compress / HashAlgorithm actual file tests

Closes #423