docs: rules for using assert, test the -O guard

[ThomasWaldmann]

Refs #8649.

The assert policy from #8649 only existed in the issue body, so asserts get converted one module at a time without a written criterion. This adds the rules to the development docs and a regression test for the startup guard (noted as missing in #10019).

Docs: new "Using assert" section in docs/development.rst

• assert only for conditions that can be false only through a borg bug: internal invariants, pre-/postconditions between borg's own functions, argument types from borg's own callers, unreachable branches.
• Everything that can be false without a borg bug uses if ...: raise ...: repository/store objects (authenticated does not mean well-formed), local cache/config/key files, legacy borg 1.x data, remote responses, user input, OS/filesystem results. Prefer borg's exception classes (IntegrityError, Error subclasses with an exit code); ValueError/TypeError only where callers catch them.
• Validate at the boundary where the data enters; deeper helpers may keep asserts as invariants.
• Never use assert for security decisions, and no side effects inside an assert expression.
• borg refuses to start with assertions disabled; the guard at the top of src/borg/archiver/__init__.py only covers code paths that import borg.archiver.
• The test suite uses plain pytest assert.

Test

test_refuses_to_run_without_assertions in src/borg/testsuite/archiver/version_cmd_test.py runs python -O -m borg --version and PYTHONOPTIMIZE=1 python -m borg --version and checks for rc 2, the "Borg requires working assertions" message on stderr and empty stdout.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.32%. Comparing base (2155628) to head (0638531).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10521      +/-   ##
==========================================
- Coverage   89.34%   89.32%   -0.02%     
==========================================
  Files         104      104              
  Lines       20079    20079              
  Branches     3152     3152              
==========================================
- Hits        17940    17936       -4     
- Misses       1481     1484       +3     
- Partials      658      659       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.