GeezerVison

Chat search now matches PR numbers by prefix, so typing 3028 already finds the chat for PR 30281 instead of showing zero results mid-typing.

The free-text search: branch in GetChats switches from pr_number = @search::bigint to pr_number::text LIKE @search || '%', still guarded by a digits-only regex so the value can never carry LIKE wildcards. The lookup stays a correlated per-chat EXISTS on the chat_diff_statuses primary key, which leads with chat_id, so the cost is the same as before and no new index is needed. The explicit pr: filter stays exact, and ordering is unchanged (no exact-match ranking) so cursor pagination is unaffected.

Behavior note: values with leading zeros (030281) no longer match PR 30281, since the old bigint cast normalized them. PR numbers never have leading zeros, so this seemed fine.

Closes CODAGT-1342

Testing

• TestGetChatsSearch gains prefix (4 -> PR 42), no-infix (2 -> none), and LIKE-wildcard (4% -> none) cases; the prefix case fails on the old predicate.
• Dogfood UAT through Coder Agents on the branch: searching 3, 302, 3028, 30281 all surface the PR-linked chat; 0281 and 302810 do not; pr:3028 stays exact.