[CSharp] Add ExampleMockValueBuilder for test/sample generation (M1)

[radhgupta]

This pull request introduces a new utility class, ExampleMockValueBuilder, to the codebase. This class provides functionality for generating mock InputOperationExample instances for operations that lack explicit examples in the specification. The builder covers a wide range of parameter and type scenarios, ensuring comprehensive and realistic mock data for testing or documentation purposes.

New mock example generation utilities:

• Added ExampleMockValueBuilder class in Microsoft.TypeSpec.Generator.Samples namespace, which generates mock examples for operations without provided examples, supporting both "ShortVersion" (required parameters only) and "AllParameters" (all parameters) variants.
• Implemented detailed logic to generate example values for all supported input types, including primitives, arrays, dictionaries, enums, models (with cycle detection), unions, literals, and special encodings (e.g., UUID, date/time, durations).
• Handles constant, default, and endpoint parameters with appropriate mock values, and supports model inheritance and discriminators for complex types.

Design doc

https://gist.github.com/radhgupta/9fee69190f1ac2252a88dd88f99de327

This is M1 of the test/sample generation milestones. Next: M2 (Expression Converter).

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