Add missing Spector scenario test coverage for C# emitter

[Copilot]

Several Spector scenarios had no corresponding NUnit test methods in the C# emitter test suite, leaving coverage gaps that would go undetected.

New tests added (31 total)

SpecialWords — ModelProperties (2)

• ModelProperties_DictMethodsAsync — dict method names as model properties
• ModelProperties_WithListAsync — list as a model property name

Payload/Multipart — FormData (5)

• WithWireName — multipart body with wire name mappings
• OptionalParts — optional fields; sends 3 requests (id-only, image-only, both)
• FileUploadFileSpecificContentType, FileUploadFileRequiredFilename, FileUploadFileArray — FormDataFile sub-client scenarios

Routes/PathParameters — Expansion types (18)

Added Standard + Explode × {primitive, array, record} for each of:

• SimpleExpansion, PathExpansion, MatrixExpansion

Routes/QueryParameters — QueryContinuation Explode (3)

• QueryContinuationExplodePrimitive/Array/Record — marked [Ignore] consistent with existing QueryContinuation Standard tests (typespec/issues/5139)

Payload/Xml (1)

• GetXmlErrorValue — verifies a ClientResultException with status 400 on the error endpoint

Type/Property/AdditionalProperties (2)

• SpreadRecordNonDiscriminatedUnion3Get/Put — additional properties typed as WidgetData2[] | WidgetData1; get-side parses the JSON array via JsonDocument, put-side writes the array as raw BinaryData

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implement missing spector scenarios for csharp emitter</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement missing spector scenarios for csharp emitter #9961

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