[PR #5344/bff76313 backport][3.11] Fix StreamResponse.prepared not returning True after EOF is sent

[patchback]

This is a backport of PR #5344 as merged into master (bff7631).

Fixes #5343

I renamed a few tests (i think the prepared attribute used to be named started and the tests were not renamed?)
And I removed some asserts around prepare in a test___repr__ test - to move them to a dedicated test

[codecov]

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Additional details and impacted files

@@           Coverage Diff           @@
##             3.11    #8894   +/-   ##
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  Coverage        ?   97.70%           
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  Files           ?      108           
  Lines           ?    34202           
  Branches        ?     4064           
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  Hits            ?    33417           
  Misses          ?      586           
  Partials        ?      199           

Flag	Coverage Δ
CI-GHA	97.60% <100.00%> (?)
OS-Linux	97.27% <100.00%> (?)
OS-Windows	94.82% <100.00%> (?)
OS-macOS	96.97% <100.00%> (?)
Py-3.10.11	97.05% <100.00%> (?)
Py-3.10.14	96.98% <100.00%> (?)
Py-3.11.9	97.22% <100.00%> (?)
Py-3.12.4	94.63% <100.00%> (?)
Py-3.12.5	97.02% <100.00%> (?)
Py-3.9.13	96.96% <100.00%> (?)
Py-3.9.19	96.89% <100.00%> (?)
Py-pypy7.3.16	96.48% <100.00%> (?)
VM-macos	96.97% <100.00%> (?)
VM-ubuntu	97.27% <100.00%> (?)
VM-windows	94.82% <100.00%> (?)

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