[http-client-csharp] Add preserveUnderscores option to ToIdentifierName and ToVariableName

[Copilot]

ToIdentifierName unconditionally treats underscores as word separators, stripping them and PascalCasing adjacent segments (Tls_1_0 → Tls10). Enum members and other identifiers that intentionally contain underscores need a way to opt out.

Changes

• IsWordSeparator: accepts preserveUnderscores param; when true, _ is no longer a word separator
• ToIdentifierName: new optional bool preserveUnderscores = false — fully backward compatible
• ToVariableName: forwards the same parameter
• Tests: 24 test cases covering default behavior and underscore preservation

Usage

// Default — existing behavior unchanged
"Tls_1_0".ToIdentifierName()                              // "Tls10"

// Preserve underscores for enum members, client names, etc.
"Tls_1_0".ToIdentifierName(preserveUnderscores: true)     // "Tls_1_0"
"TLS_1_0".ToIdentifierName(preserveUnderscores: true)     // "TLS_1_0"

Original prompt

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This section details on the original issue you should resolve

<issue_title>[http-client-csharp] ToIdentifierName is too strict</issue_title>
<issue_description>We have a ToIdentifierName and ToVariableName which converts the input names from typespec into our C# identifiers in generated code.
Now, these two methods will trim off underscores in the name.
But in some cases, we need underscore to stay in the identifier, the most common case is the name of an enum member, such as Tls_1_0.

Can we update this method to make it accept an extra argument so that from downstream we could allow to add clientName to enum members to rename them to a name with underscore?</issue_description>

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

• Fixes [http-client-csharp] ToIdentifierName is too strict #10053

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