fix(op_crates/fetch): correct regexp for fetch header (#8927)

Fix bug in regular expression and make the regular expression more
strict.

In a string passed to new RegExp(), '[\t\s]' is identical to '[ts]' and
not `/[\t\s]/`. For that, the backslash needs to be escaped in the
string. Futhermore, `\t` is the tab character and is included in the
special regexp value `\s` so is unnecessary.

That would reduce the RegExp to new RegExp(`^${value}\\s*;?`) but
there's no point in matching 0 or more space characters followed by 0 or
one semi-colons as that will match no matter what follows `value`.

To make it more strict, require one of space, semicolon, or
end-of-string after value.

cli/tests/unit/fetch_test.ts

@@ -249,6 +249,25 @@ unitTest(
   },
 );
 
+unitTest(
+  { perms: { net: true } },
+  async function fetchMultipartFormBadContentType(): Promise<void> {
+    const response = await fetch(
+      "http://localhost:4545/multipart_form_bad_content_type",
+    );
+    assert(response.body !== null);
+
+    await assertThrowsAsync(
+      async (): Promise<void> => {
+        await response.formData();
+      },
+      TypeError,
+      "Invalid form data",
+    );
+    await response.body.cancel();
+  },
+);
+
 unitTest(
   { perms: { net: true } },
   async function fetchURLEncodedFormDataSuccess(): Promise<void> {