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/paste.fish

@@ -11,6 +11,9 @@
 #   Outputs clipboard contents to stdout. Uses wl-paste on Wayland, xclip on
 #   X11, or win32yank on WSL2.
 #
+# DEPENDENCIES
+#   _fish_clipboard_paste
+#
 # ARGUMENTS
 #   args...  Arguments forwarded to the clipboard tool
 #
@@ -24,14 +27,5 @@
 # EXAMPLE
 #   paste > file.txt
 function paste --description 'Paste from clipboard'
-    if type -q wl-paste
-        wl-paste $argv
-    else if type -q xclip
-        xclip -selection clipboard -o $argv
-    else if type -q win32yank.exe
-        win32yank.exe -o --lf $argv
-    else
-        echo "Error: No clipboard provider (wl-paste, xclip, or win32yank) found." >&2
-        return 1
-    end
+    _fish_clipboard_paste $argv
 end