fix(server): Track status for content-length terminated bodies

[jan-auer]

Streaming responses that declare a content-length were recorded as
client disconnects (499) in server.requests.duration, a regression
from #555. Hyper stops polling a response body once the declared number
of bytes has been written, so the wrapper never saw an end-of-stream
poll and its guard fell back to 499 when dropped.

MetricsBody now counts the bytes it forwards and completes the request
once the declared length is reached, alongside the existing
end-of-stream and trailers cases. Responses whose body hyper never polls
at all are completed up front when they are wrapped.

The tests were reworked around a helper that serves a real axum handler
through the middleware and returns the tracked status and duration,
which made room for a paused-clock test asserting the duration spans
body streaming rather than ending at the response headers. Capturing
metrics across await points needs a new
with_capturing_test_client_async in objectstore-metrics.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.62%. Comparing base (5ea168b) to head (ccda332).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
- Coverage   87.69%   87.62%   -0.08%     
==========================================
  Files          94       94              
  Lines       15510    15591      +81     
==========================================
+ Hits        13601    13661      +60     
- Misses       1909     1930      +21     

Components	Coverage Δ
Rust Backend	92.25% <100.00%> (-0.13%)	⬇️
Rust Client	79.89% <ø> (ø)
Python Client	90.98% <ø> (+0.07%)	⬆️

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[lcian]

Looks good to me, nice refactor of the tests.