[api-documenter] Add support for @defaultValue in Markdown and Yaml documenters

[colinaaa]

Summary

From the perspective of an end user, the @defaultValue tag on class properties, variables, methods, and functions was previously extracted by api-extractor but ignored by api-documenter, resulting in missing default value documentation. This change adds the logic to both MarkdownDocumenter and YamlDocumenter to extract and render the @defaultValue tag correctly.

Fixes #4249

Details

• MarkdownDocumenter: Added _writeDefaultValueSection to render a "Default Value" heading and block content. It is now called for Properties, PropertySignatures, Variables, TypeAliases, Methods, MethodSignatures, Constructors, ConstructSignatures, and Functions.
• YamlDocumenter: Extracted the default value from customBlocks and serialized it to yamlItem.defaultValue.
• Schemas: Updated IYamlItem, typescript.schema.json, and ISDPYamlFile to formally support defaultValue?: string, mapping it correctly through the UDP to SDP pipeline in ToSdpConvertHelper.ts.
• Regenerated test snapshots for DocClass1.ts to verify output generation across both Markdown and Yaml formats.

How it was tested

Added @defaultValue "Hello World" and @defaultValue 123 to test fixtures in build-tests/api-documenter-test. Ran heft test and verified that the newly generated .snap snapshots properly contain the "Default Value" headings in Markdown and the defaultValue keys in the DocFX YAML payloads.