Support XML serialization/deserialization

[joheredi]

Summary

This PR adds full XML serialization and deserialization support for the TypeSpec TypeScript emitter, enabling generated clients to handle XML request and response bodies.

Changes

New Static Helpers

• xml-helpers.ts - Runtime XML serialization/deserialization utilities using fast-xml-parser
  • serializeToXml() - Converts TypeScript models to XML strings
  • deserializeXml() - Parses XML responses into TypeScript models
  • Support for XML attributes, namespaces, wrapped/unwrapped arrays, dictionaries, and text content
  • Proper whitespace preservation in text nodes

Code Generation

• buildXmlSerializerFunction.ts - New serializer generator for XML operations
• operationHelpers.ts - Extended to detect XML content types and generate appropriate serialization calls
• emitModels.ts - Updated to emit XML serialization metadata
• buildOperations.ts - Updated operation generation for XML handling

Dependencies

• Added fast-xml-parser dependency for XML parsing/building

Test Coverage

• 24 integration tests covering all Payload.Xml spector test scenarios from the spec:

  • SimpleModel (GET/PUT)
  • ModelWithSimpleArrays (GET/PUT)
  • ModelWithArrayOfModel (GET/PUT)
  • ModelWithOptionalField (GET/PUT)
  • ModelWithAttributes (GET/PUT)
  • ModelWithUnwrappedArray (GET/PUT)
  • ModelWithRenamedArrays (GET/PUT)
  • ModelWithRenamedFields (GET/PUT)
  • ModelWithEmptyArray (GET/PUT)
  • ModelWithText (GET/PUT)
  • ModelWithDictionary (GET/PUT)
  • ModelWithEncodedNames (GET/PUT)

• 54 unit tests for the XML helper functions covering:

  • Primitive serialization (strings, numbers, booleans, dates, bytes)
  • Array serialization (wrapped and unwrapped)
  • Attribute handling
  • Namespace support
  • Dictionary serialization/deserialization
  • Content type detection
  • Round-trip serialization
  • Edge cases (empty arrays, null values, special characters)

XML Features Supported

Feature	Serialization	Deserialization
Simple models	✅	✅
Nested models	✅	✅
Wrapped arrays	✅	✅
Unwrapped arrays	✅	✅
XML attributes	✅	✅
XML namespaces	✅	✅
Dictionary types	✅	✅
Text content	✅	✅
Renamed elements	✅	✅
Date encoding (RFC3339, RFC7231, Unix)	✅	✅
Base64 bytes	✅	✅

Breaking Changes

None

[jeremymeng]

note: storage needs to configure the xml char key because the default one is _ which is an allowed character in storage identifiers metadata. https://github.com/Azure/azure-sdk-for-js/blob/3c50f9567bd325c7b3264ac02442685849f2bbfc/sdk/storage/storage-blob/src/Pipeline.ts#L296

[joheredi]

note: storage needs to configure the xml char key because the default one is _ which is an allowed character in storage identifiers metadata. https://github.com/Azure/azure-sdk-for-js/blob/3c50f9567bd325c7b3264ac02442685849f2bbfc/sdk/storage/storage-blob/src/Pipeline.ts#L296

fast-xml-parser seems to use #text so it wouldn't collide with storage. I added test cases for this to validate this.