Improve the logic for detecting network availability issues

[jamescrosswell]

See:

• https://github.com/getsentry/sentry-dotnet/pull/3938/files#r1945562033

sentry-dotnet/src/Sentry/Internal/Http/CachingTransport.cs

Lines 303 to 309 in 2529ab5

	private static bool IsNetworkUnavailableError(Exception exception) =>
	exception switch
	{
	// TODO: Could we make this more specific? Lots of these errors are unrelated to network availability.
	HttpRequestException or WebException or IOException or SocketException => true,
	_ => false
	};

Some of the exception types we use to detect network availability issues are umbrella's for lots of potential root cause errors.

HttpRequestException, for example, has a StatusCode that could indicate any number of reasons why the request failed. Only a few of these would indicate network availability issues.

Similarly, the SocketError enum indicates all kinds of different root causes for a socket error.

This is complicated by the fact that these more specific codes appear to depend on the operating system where the HttpClient is running... so we'd need different code for different operating systems (at the very least, Windows + Others... hopefully Unix and macOS do more or less the same thing).

This is unrelated to this PR btw - I just noticed it when trying to identify, more specifically, a broken pipe exception.