refactor(clipboard): share provider detection across y/p/paste/hist

Extracts _fish_clipboard_copy and _fish_clipboard_paste so the
wl-copy/xclip/win32yank fallback chain lives in one place instead of
four near-duplicates. hist now goes through the same chain, so it
also gets the xclip (X11) fallback it was missing before, alongside
win32yank on WSL2.

functions/p.fish

@@ -11,6 +11,9 @@
 #   Outputs clipboard contents to stdout. Uses wl-paste on Wayland, xclip on
 #   X11, or win32yank on WSL2. Supports -h/--help for usage info.
 #
+# DEPENDENCIES
+#   _fish_clipboard_paste
+#
 # ARGUMENTS
 #   -h, --help  Show usage help
 #   args...     Arguments forwarded to the clipboard tool
@@ -41,19 +44,5 @@ function p --description 'Put from clipboard'
         return 0
     end
 
-    # Determine the clipboard provider
-    set -l paste_cmd
-    if type -q wl-paste
-        set paste_cmd wl-paste
-    else if type -q xclip
-        set paste_cmd xclip -selection clipboard -o
-    else if type -q win32yank.exe
-        set paste_cmd win32yank.exe -o --lf
-    else
-        echo "Error: No clipboard provider (wl-paste, xclip, or win32yank) found." >&2
-        return 1
-    end
-
-    # Execute the paste command with any provided arguments
-    $paste_cmd $argv
+    _fish_clipboard_paste $argv
 end