zsh: prevent duplicate default REMAINDER spec in generated completions

[ThomasWaldmann]

Fixes #183.

When completing commands with a subparser that uses nargs=REMAINDER, zsh could
emit:

_arguments:comparguments:327: doubled rest argument definition: *::: :->repro

Root cause:

• The zsh template appended the default positional/rest specs (': :{prefix}_commands' and
  '*::: :->{name}') on every function invocation. It only checked for generic
  variadic/remainder tokens ((*), (-)*) and not whether the exact default entry
  (*::: :->{name}) had already been added. Repeated invocations led to duplicated
  rest specs and the _arguments error above.

Fix:

• Add a per-function guard variable {prefix}_defaults_added, initialized alongside
  {prefix}_options and set to 1 after the first append. This guarantees that the
  default '*::: :->{name}' is added at most once per session.
• Keep (and extend) the presence checks: only append the defaults if neither a
  variadic/rest token ((*), (-)*) nor the exact default entry is already present.

Impact:

• Eliminates duplicated *::: :->… entries and the “doubled rest argument definition”
  error in zsh, including across multiple invocations and after re-sourcing the file.
• Generated zsh output is unchanged except for the guard variable lines.

[ThomasWaldmann]

Maybe I could even fix #183 with this, Junie AI was helpful.

I manually tested it, it works. But carefully review, I have no idea what I am doing here.

[ThomasWaldmann]

That "thesis" commit comment was also written by Junie, btw. :-)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.18%. Comparing base (acc76fd) to head (171be68).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #200   +/-   ##
=======================================
  Coverage   89.18%   89.18%           
=======================================
  Files           3        3           
  Lines         370      370           
=======================================
  Hits          330      330           
  Misses         40       40           

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[casperdcl]

lgtm; wdyt @Freed-Wu @skshetry?

[ThomasWaldmann]

Oops, I hope I didn't kill anything here by force-pushing.
I rebased this on current main and also adapted the failing test.

[casperdcl]

just force-pushed:

• human-written commit msg
• fix tests
• fix flake8