completion(zsh): use private mktemp dir for completion FIFO

FIFO path was predictable (${TMPDIR:-/tmp}/fzf-complete-fifo-$$) and
_fzf_feed_fifo ignored rm/mkfifo failures, so on a shared /tmp another
user could pre-create the path (symlink or FIFO) and have the victim's
redirect follow it.

Allocate the FIFO inside a per-invocation mktemp -d directory, check
mkfifo status, and clean up the directory instead of the bare path.

Reported-by: sb24CK (https://github.com/sb24CK)

CHANGELOG.md

@@ -13,6 +13,8 @@ CHANGELOG
     - On tmux 3.7, the window is still unzoomed, as tmux 3.7b crashes when a floating pane is created over a zoomed window
 - Security fixes
     - fzf-tmux no longer evaluates an attached layout size (e.g. `-l50`) via bash arithmetic, which could run commands or inject tmux flags (GHSA-w6x2-f5m4-xw82)
+    - zsh completion now creates its FIFO in a private `mktemp -d` directory instead of a predictable `/tmp` path (GHSA-7rj6-7pqx-v7rm)
+    - Reported by Sai Sreewathsa Kovalluri, Shon Babu, and Kannan K (Researchers) of Innspark Solutions
 
 0.74.4
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