fix: improve tcsh completion support

[ThomasWaldmann]

This commit fixes critical evaluation crashes in the generated tcsh completion script and significantly cleans up the Python generation logic:

• monkeypatch shtab to natively support positional completion under subcommands for tcsh (e.g., borg help <topic>).
• fix "if: Empty if." errors in tcsh by injecting array bounds checks ($#cmd >= max_idx) inside the monkeypatched shtab generator.
• fix recursive parser crashes in tcsh by replacing unescaped nested backticks (...) with safe eval evaluations.
• deduplicate tcsh fallback rules to reduce the script payload size, preventing tcsh memory buffer truncations that led to cryptic "Illegal variable name." exceptions.
• no support yet for archive, aid:, and tags completion features for tcsh

Description

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[ThomasWaldmann]

This was created with AI assistance (Gemini Flash/Pro) and likely needs more work:

• that monkeypatching of shtab isn't pretty. IF there are bugs / missing features in shtab tcsh support, they should be fixed there.
• archivename / aid completion is missing. The AI tried to write that, but ran into severe troubles. To at least have something simpler to begin with, I decided to skip that for the initial commit.

[codecov]

Codecov Report

❌ Patch coverage is 90.24390% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.40%. Comparing base (1356aeb) to head (952d70a).
⚠️ Report is 13 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archiver/completion_cmd.py	90.24%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9503      +/-   ##
==========================================
+ Coverage   83.36%   83.40%   +0.04%     
==========================================
  Files          87       87              
  Lines       15439    15523      +84     
  Branches     2309     2336      +27     
==========================================
+ Hits        12870    12947      +77     
- Misses       1818     1821       +3     
- Partials      751      755       +4     

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[ThomasWaldmann]

@simaoafonso-pwt Could you have a look at that monkeypatch please?