Fix redirect response bodies being written to response_stream

[IanButterworth]

Note, written by Claude, hence the draft.

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Fixes #1165

Problem

When using response_stream with HTTP requests that involve redirects, both the redirect response body AND the final response body were being written to the stream. This is incorrect behavior - only the final response body should be written to the stream, and redirect response bodies should be ignored (similar to how curl handles redirects with "Ignoring the response-body").

Root Cause

The issue was introduced in the version range v1.5.5...v1.6.3, specifically related to changes in PR #975 that fixed StatusError exception handling. While that fix was correct and necessary, it highlighted an existing issue in StreamRequest.jl:

• Redirect responses (3xx status codes) return false for iserror(res) but true for isredirect(res)
• In readbody!(), they were going through the normal "success" path and writing their bodies to response_stream
• When redirects are followed, these intermediate response bodies should be discarded, not written to the stream

Solution

Modified StreamRequest.jl to:

1. Added willredirect() function that determines if a redirect response will actually be followed (using the same logic as RedirectRequest layer)
2. Updated the condition in readbody!() from !iserror(res) to !iserror(res) && !willredirect(res)
3. Redirect responses that will be followed are now treated like error responses - their bodies are read but stored in request context instead of written to response_stream

Behavior

• ✅ Redirect followed: Only final response body written to response_stream
• ✅ Multiple redirects: Only final response body written to response_stream
• ✅ redirect=false: Redirect response body written to response_stream (correct)
• ✅ Redirect limit reached: Final redirect response body written to response_stream (correct)
• ✅ No redirects: Normal response body written to response_stream (unchanged)

Testing

Added comprehensive test case in client.jl that covers:

• Basic redirect with response_stream
• Multiple redirects with response_stream
• Redirect disabled (redirect=false) with response_stream

The fix ensures HTTP.jl now matches the expected behavior described in the issue and aligns with how other HTTP clients (like curl) handle redirect response bodies in streaming mode.