fix(snippetz): escape strings in C# HttpClient code examples - #10503

Problem

The C# HttpClient code example writes some values into C# string literals without escaping them, so the copied code does not compile or sends different data:

• A URL, header name or value, cookie, Authorization scheme or token, or form field that contains a double quote or backslash breaks the string literal.
• Octet-stream bodies escaped quotes but not backslashes or line breaks, so a body with \ or a new line was written wrongly.
• The same goes for Accept values and multipart content types.

Solution

Use the existing escapeCSharpString helper for these values, and make it escape carriage returns and new lines too. JSON and text bodies still use the raw string literal, so their output is unchanged.

I added tests for a URL, headers, cookie and form fields with quotes and backslashes, and for an octet-stream body with a backslash and a line break. Both fail without the fix.

Checklist

• I explained why the change is needed.
• I added a changeset.
• I added tests.
• I updated the documentation. (not needed, no docs describe this)