fix(core): Refactor copy file

[Tranquility2]

Follow-up to #852, cleaning up the copy file feature after it landed.
(Related to #676)

A few things stood out:

• _transfer_file_content_into_container and _transfer_directory_into_container did basically the same thing (build a tar, upload it) but were separate methods with duplicated logic
• assert was used for error handling in production code, these silently vanish under python -O, so real failures would just pass through unnoticed
• copy_from_container treated each HTTP chunk from get_archive() as its own tar file, works fine for small files that fit in one chunk, breaks silently for anything larger
• The tar-building logic was stuffed inside DockerContainer when it really has nothing to do with container lifecycle, it's just data transformation on a Transferable

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.24%. Comparing base (898faf6) to head (6a621a4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
core/testcontainers/core/container.py	46.15%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
- Coverage   79.54%   79.24%   -0.30%     
==========================================
  Files          15       15              
  Lines        1281     1277       -4     
  Branches      151      154       +3     
==========================================
- Hits         1019     1012       -7     
- Misses        218      219       +1     
- Partials       44       46       +2     

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[alexanderankin]

??

[alexanderankin]

we should update this with release-please

[alexanderankin]

oh this is unsolved problem. what happens if we ignore it? i guess uv will fail to install.

[Tranquility2]

I think it was somehow missed 🤷