refactor: audit zero-coverage and unreachable code

[ReubenBond]

Goal

Classify zero-coverage source and unsupported-code paths, then remove dead code, simplify unreachable branches, or add focused contract tests. Denominator reductions must represent real maintenance reduction rather than metric gaming.

Coverage report: https://github.com/dotnet/orleans/runs/98679059570

Evidence

The merged report contains approximately:

• 155 entirely uncovered files with more than 4,300 executable line entries in the method-level report.
• 458 methods with CRAP scores above 30.
• 7,197 methods below the 90% line / 80% branch analysis thresholds.
• 127 explicit NotImplementedException or NotSupportedException sites, many uncovered.

Strong investigation candidates include:

• MessagingEvents.cs: conditionally compiled emitters with no discovered MESSAGING_TRACE definition.
• IJournaledState.DeepCopy() implementations which throw and have no discovered production caller.
• Orleans.TestingHost/Utils/StorageEmulator.cs: legacy Windows Azure Storage Emulator integration while CI uses Azurite.
• Unreferenced compiler helpers such as MethodSignatureComparer and unused instance/stream hashing APIs.
• Shared/linked source whose repeated assembly entries distort method-level coverage.

Known reachable/intentional paths must not be removed solely because they are uncovered, including stream producer callbacks, codec rejection contracts, public compatibility APIs, and extension-point virtual methods.

Work

• Inventory every entirely uncovered maintained source file.
• Inventory explicit throw sites and non-API [ExcludeFromCodeCoverage] uses.
• Classify each item as dead, generated/reference-only, platform-specific, intentional unsupported behavior, shipped test support, or missing tests.
• Require call-site, reflection/source-generation, public API, and package compatibility evidence before removal.
• Remove dead private/internal code and obsolete infrastructure.
• Add negative tests and clear contracts for intentional unsupported paths.
• Narrowly exclude only mechanically generated or structurally untestable code, with rationale and configuration tests.
• Report removed/excluded lines separately from newly covered lines.

Acceptance criteria

• Every zero-coverage maintained file and explicit throw site has a recorded classification.
• Confirmed dead code is removed with build/API validation.
• Intentional unsupported behavior has focused tests.
• Coverage exclusions remain narrow, reviewable, and protected against broadening.
• The resulting coverage increase can be decomposed into test gains, code removal, and approved scope corrections.