test: Round 3 — 50 tests for Download orchestrator, HttpDownloadExecutor, VersionService, SilentPollOrchestrator (closes #427)

[JusterZhu]

Summary

Final round of the unit test initiative. Adds 50 tests across 4 remaining modules.

New Files (4 files)

Module	File	Tests
Download	DefaultDownloadOrchestratorTests.cs	7
	HttpDownloadExecutorTests.cs	9
Network	VersionServiceRetryTests.cs	9
Silent	SilentPollOrchestratorTests.cs	10

Coverage

Download (16 tests)

• DefaultDownloadOrchestrator: Ctor (null guard/custom options/custom policy), ExecuteAsync (null plan/empty plan/directory creation/URI filename)
• HttpDownloadExecutor: Ctor (null guard/defaults/resume/custom timeout), ExecuteAsync (200 success/500 error/404 not found/resume append/overwrite without resume) plus MockHttpMessageHandler

Network (9 tests)

• VersionService.IsRetryable: OperationCanceledException→false, TaskCanceledException→false, TimeoutException→true, IOException→true, HttpRequestException without timeout→false, InvalidOp/NRE→false
• HttpClientProvider: singleton identity, not null

Silent (10 tests)

• SilentPollOrchestrator: Ctor null guards, WithHooks/WithReporter chain pattern, Stop/Dispose no-throw
• SilentOptions: defaults (1hr poll, auto-install=false), custom values

Test Results

\
Passed: 639 | Failed: 0 | Total: 639
\\

Three-Round Cumulative

Round	Issue	PR	New Tests	Total
#1	#423	#424	+538	538
#2	#425	#426	+62	600
#3	#427	#428	+39	639

Closes #427