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.

src/complete.rs

@@ -32,8 +32,7 @@
 use fish_util::wcsfilecmp;
 use fish_wcstringutil::{
     StringFuzzyMatch, string_fuzzy_match_string, string_prefixes_string,
-    string_prefixes_string_case_insensitive, string_suffixes_string_case_insensitive,
-    strip_executable_suffix,
+    string_suffixes_string_case_insensitive, strip_executable_suffix,
 };
 use fish_widestring::{WExt as _, charptr2wcstring};
 use std::{
@@ -160,12 +159,14 @@ pub fn new(
         flags: CompleteFlags,
     ) -> Self {
         let flags = resolve_auto_space(&completion, flags);
-        Self {
+        let zelf = Self {
             completion,
             description,
             r#match,
             flags,
-        }
+        };
+        assert!(!zelf.r#match.requires_full_replacement() || zelf.replaces_token());
+        zelf
     }
 
     pub fn from_completion(completion: WString) -> Self {
@@ -1508,7 +1509,7 @@ fn complete_param_for_command(
                     if !self.completions.add(Completion::new(
                         completion,
                         o.desc.localize().to_owned(),
-                        StringFuzzyMatch::exact_match(),
+                        r#match,
                         flags | CompleteFlags::NO_SPACE,
                     )) {
                         return false;
@@ -1519,7 +1520,7 @@ fn complete_param_for_command(
                 if !self.completions.add(Completion::new(
                     whole_opt.slice_from(offset).to_owned(),
                     o.desc.localize().to_owned(),
-                    StringFuzzyMatch::exact_match(),
+                    r#match,
                     flags,
                 )) {
                     return false;