[WinHTTP] Validate header values for ASCII

[Copilot]

WinHttpHandler was passing headers to WinHTTP.dll without validating that header values contain only ASCII characters, unlike SocketsHttpHandler which performs this validation.

Changes

Added ASCII validation to WinHttpHandler.AddRequestHeaders():

• Added IsAscii() helper method to check for ASCII characters (char <= 127)
• Added ValidateHeadersForAscii() method that throws HttpRequestException for non-ASCII headers
• Modified header serialization to validate:
  • Cookie headers from WinHttpCookieContainerAdapter.GetCookieHeader()
  • General request headers from requestMessage.Headers.ToString()
  • Content headers from requestMessage.Content.Headers.ToString()

Added comprehensive tests:

• SendAsync_RequestWithNonAsciiHeaderValue_ThrowsHttpRequestException() - validates rejection of non-ASCII request headers
• SendAsync_RequestWithAsciiHeaderValue_Succeeds() - validates ASCII headers work normally
• SendAsync_RequestWithNonAsciiContentHeader_ThrowsHttpRequestException() - validates rejection of non-ASCII content headers

Behavior

Now throws HttpRequestException with message "Request headers must contain only ASCII characters." when header values contain characters > 127, matching the behavior and security posture of SocketsHttpHandler.

Testing

• All 3 new tests pass ✅
• All existing 175 unit tests continue to pass ✅
• Validates RFC 9110 compliance for header values

Fixes #115112.

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[stephentoub]

The exception message should be in the resx.

[Copilot]

Moved the hardcoded exception message to the Strings.resx file and updated the code to use SR.net_http_headers_must_be_ascii. The message is now properly localized. 00a1e04

[stephentoub]

@ManickaP, is restricting to ASCII correct, or should it support Latin-1?

[ManickaP]

Yes, for historical reasons it should allow Latin-1. On the other hand, it should not allow "dangerous" controls like CR LF and NUL.

[Copilot]

Updated header validation to allow Latin-1 characters (0-255) instead of just ASCII (0-127) for historical reasons, while still blocking dangerous control characters (CR, LF, NUL). Changed validation approach to check individual header values rather than the serialized header string to avoid issues with protocol formatting. Commit ab6c173.

[ManickaP]

After a discussion with @MihaZupan, closing this in favor of #116335.