[EnesKartal]

Fixes #683.

ConditionalBlockAccumulatorContext handled every chained {{else ...}} itself, but only recognised a two-argument {{else if x}}:

• {{#if a}}A{{else ifEq b "x"}}B{{else}}C{{/if}} took {{else ifEq b "x"}} as a plain {{else}}, so the next {{else}} built Expression.IfThen(null, ...) and Compile threw ArgumentNullException (Parameter 'test').
• Without the final {{else}} the ifEq condition was silently dropped and B always rendered.
• A two-argument {{else with b}} was evaluated as {{else if b}}.
• {{else unless x}} under {{#if}} was evaluated as {{else if x}} (the condition was not negated).

Change

• HandlesChainedElseInternally becomes HandlesChainedElse(item). The if/unless context returns true only for {{else if ...}} and {{else unless ...}}. Any other {{else name ...}} goes through the desugaring from feat: support else-if chaining for block helpers #648, the same as under other block helpers.
• {{else unless x}} is negated. {{else if}} / {{else unless}} with the wrong number of arguments now throw HandlebarsCompilerException ("#if requires exactly one argument"), like {{#if}} / {{#unless}} since fix: throw HandlebarsCompilerException for #if/#unless with wrong argument count #678.
• The desugaring returns AccumulatedBlock without passing the closing tag to the outer context, so the conditional context now builds its last branch in AccumulatedBlock from the buffer instead of in IsClosingElement. The result is the same expression as before for templates that were already supported.

Test plan

New tests next to the #263 chained-else tests in IssueTests, plus two InlineData rows in ExceptionTests.TestConditionalBlockWithWrongArgumentCountException.

Before the fix (dotnet test --filter ..., library changes stashed):

Failed ExceptionTests.TestConditionalBlockWithWrongArgumentCountException(source: "{{#if a}}yes{{else unless b c}}no{{/if}}", helper: "#unless")
Failed ExceptionTests.TestConditionalBlockWithWrongArgumentCountException(source: "{{#if a}}yes{{else if b c}}no{{/if}}", helper: "#if")
Failed IssueTests.ElseWithCustomBlockHelperInvocationChainsUnderIf          (ArgumentNullException: Value cannot be null. (Parameter 'test'))
Failed IssueTests.ElseWithCustomBlockHelperInvocationUnderIfKeepsItsCondition
Failed IssueTests.ElseWithSingleArgumentBlockHelperUnderIfIsNotTreatedAsElseIf
Failed IssueTests.ElseUnlessChainsUnderIf                                   (Expected "B", Actual "C")
Failed IssueTests.ElseWithCustomBlockHelperInvocationChainsUnderUnless      (ArgumentNullException)
Failed!  - Failed: 7, Passed: 10, Total: 17

After the fix, whole test project:

Passed!  - Failed: 0, Passed: 2241, Skipped: 0, Total: 2241 - Handlebars.Test.dll (net10.0)

dotnet build source/Handlebars/Handlebars.csproj -f netstandard2.0 also builds.