Fix option substr completions not being filtered out

Commit 3546ffa3ef (reader handle_completions(): remove dead filtering code,
2026-01-02) gives a proof of correctness that still makes sense;
The first lemma ("if will_replace_token") is trivially true, so no need to
assert it.
The second lemma ("if !will_replace_token") is violated in some edge cases:
we claim that given a token "-c", the option completion "--clip" is an exact match,
which is not true, it's a substring match.

Fix that, asserting the claim.

tests/checks/tmux-complete4.fish

@@ -0,0 +1,30 @@
+#RUN: %fish %s
+#REQUIRES: command -v tmux
+#REQUIRES: uname -r | grep -qv Microsoft
+
+isolated-tmux-start -C '
+    complete : -s c -l clip
+    complete : -s q -l qrcode
+    set -g fish_autosuggestion_enabled 0
+'
+touch somefile1
+touch somefile2
+
+isolated-tmux send-keys C-l ': -c'
+
+function tab
+    isolated-tmux send-keys Tab
+    tmux-sleep
+    isolated-tmux capture-pane -p | sed '/./ { s,^,[,; s,$,], }'
+end
+
+tab
+# CHECK: [prompt 0> : -cq]
+
+tab
+# CHECK: [prompt 0> : -cq somefile]
+# CHECK: [somefile1  somefile2]
+
+tab
+# CHECK: [prompt 0> : -cq somefile1]
+# CHECK: [somefile1  somefile2]