[vim/vim] fix: ":for" over a tuple leaks memory on every iteration (PR #20914)

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Problem:  Looping over a tuple with ":for" copies each item with copy_tv() =
but never clears the copy, leaking the value on every iteration (26MB over =
100k iterations of a two-string tuple).  Container items keep an extra refe=
rence forever, also defeating garbage collection.

Solution: Clear the copied typval on both return paths, like the string bra=
nch of next_for_item() already does.

Disclosure: Claude Fable 5 was used to assist with finding this bug. I revi=
ewed this, but I am new to this codebase so forgive me if I am wrong about =
this problem. This fix was also verified with tests.
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20914

-- Commit Summary --

  * fix: ":for" over a tuple leaks memory on every iteration

-- File Changes --

    M src/eval.c (7)

-- Patch Links --

https://github.com/vim/vim/pull/20914.patch
https://github.com/vim/vim/pull/20914.diff

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<p dir=3D"auto">Problem:  Looping over a tuple with ":for" copies each item=
 with copy_tv() but never clears the copy, leaking the value on every itera=
tion (26MB over 100k iterations of a two-string tuple).  Container items ke=
ep an extra reference forever, also defeating garbage collection.</p>
<p dir=3D"auto">Solution: Clear the copied typval on both return paths, lik=
e the string branch of next_for_item() already does.</p>
<p dir=3D"auto">Disclosure: Claude Fable 5 was used to assist with finding =
this bug. I reviewed this, but I am new to this codebase so forgive me if I=
 am wrong about this problem. This fix was also verified with tests.</p>

<hr>

<h4>You can view, comment on, or merge this pull request online at:</h4>
<p>&nbsp;&nbsp;<a href=3D'https://github.com/vim/vim/pull/20914'>https://gi=
thub.com/vim/vim/pull/20914</a></p>

<h4>Commit Summary</h4>
<ul>
  <li><a href=3D"https://github.com/vim/vim/pull/20914/commits/9a4ad0117678=
e829deb614cf6d0b338e38a3c927" class=3D"commit-link">9a4ad01</a>  fix: &quot=
;:for&quot; over a tuple leaks memory on every iteration</li>
</ul>

<h4 style=3D"display: inline-block">File Changes </h4> <p style=3D"display:=
 inline-block">(<a href=3D"https://github.com/vim/vim/pull/20914/files">1&n=
bsp;file</a>)</p>
<ul>
  <li>
    <strong>M</strong>
    <a href=3D"https://github.com/vim/vim/pull/20914/files#diff-3f6891fec05=
432f415cfd49150db67e2bc72d20e0a94a4e3ac762f7ff55f9921">src/eval.c</a>
    (7)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href=3D'https://github.com/vim/vim/pull/20914.patch'>https://githu=
b.com/vim/vim/pull/20914.patch</a></li>
  <li><a href=3D'https://github.com/vim/vim/pull/20914.diff'>https://github=
.com/vim/vim/pull/20914.diff</a></li>
</ul>

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