detectPrefixQuery required the trigger char to sit at start-of-string or
after whitespace, so typing `(#channel` (or `[@name`, `{#chan`) glued to
an opening bracket never opened the autocomplete — the user had to add a
space after the bracket. Allow opening brackets ([{ as a word boundary in
both the fast-path regex and the multi-word backward scan, kept in sync via
a single isBoundaryChar definition. Fixes 7796a4f4.
Adds detectPrefixQuery.test.mjs (first test on this helper): bracket
boundaries for # and @, single- and multi-word paths, and negatives
(prefix glued to a word char still does not trigger).
Co-authored-by: npub17jjz49l9jjmhhk7cac63j8yt9z555n9cw8vk7v5jz4vzw4ppld5qgj57cc <f4a42a97e594b77bdbd8ee35191c8b28a94a4cb871d96f32921558275421fb68@sprout-oss.stage.blox.sqprod.co>
Signed-off-by: npub17jjz49l9jjmhhk7cac63j8yt9z555n9cw8vk7v5jz4vzw4ppld5qgj57cc <f4a42a97e594b77bdbd8ee35191c8b28a94a4cb871d96f32921558275421fb68@sprout-oss.stage.blox.sqprod.co>
