Fix "The request message was already sent" error when following redirects

[Claude]

Summary

A customer on HTTP/1.1 with FollowRedirects=true reported intermittent exceptions: "The request message was already sent. Cannot send the same request message multiple times." (~24/1500 requests over 8 hours).

Root cause

GetHttpRequestResponseAsync built a single HttpRequestMessage (bound to a single HttpContent) and dispatched it once. A System.Net.Http.HttpRequestMessage can only be sent one time — once sent it sets an internal flag and any re-dispatch throws this exact InvalidOperationException. When the handler follows a 3xx redirect, or when the shared cached HttpClient (default CacheHttpClient=true) internally re-dispatches under concurrency/connection churn, the same message is re-sent and the guard trips. This explains the low, environment-dependent error rate: only requests that hit a redirect or a re-dispatched connection are affected.

Fix

• Construct a fresh HttpRequestMessage and HttpContent per send attempt via a content factory (Func<HttpContent>), so an internal re-send never reuses a consumed message.
• Transparently retry (bounded to 3 attempts) only on the specific "already sent" InvalidOperationException.
• Content is now owned per attempt; removed the outer httpContent disposal in finally accordingly.
• Split the send logic into BuildRequestMessage and SendRequestMessageAsync helpers; behavior otherwise unchanged.

Testing

• Added RequestShouldFollowRedirectForRequestWithBody which POSTs a JSON body through a 307 redirect with FollowRedirects=true and asserts the body reaches the final endpoint.
• dotnet test Frends.HTTP.Request.sln: 23 passed, 0 failed, 2 skipped.

Test plan

• Build succeeds (0 errors)
• Full Request test suite passes
• New redirect-with-body test passes