fix: search usage by name or username (#23317)

## Summary

The search field on `/agents/settings/usage` previously only matched
against usernames. This updates the SQL query to also match against the
user's display name via `ILIKE`, and updates the frontend placeholder
and variable names to reflect the broader search scope.

## Changes

- **SQL** (`coderd/database/queries/chats.sql`,
`coderd/database/queries.sql.go`): Added `OR u.name ILIKE '%' ||
@username::text || '%'` to the `GetChatCostPerUser` query's WHERE
clause.
- **Frontend** (`site/src/pages/AgentsPage/SettingsPageContent.tsx`):
Renamed `usernameFilter`/`debouncedUsername` to
`searchFilter`/`debouncedSearch`, updated placeholder to "Search by name
or username".

---

PR generated with Coder Agents

coderd/database/queries.sql.go

@@ -4062,6 +4062,7 @@ WITH chat_cost_users AS (
         AND (
             $5::text = ''
             OR u.username ILIKE '%' || $5::text || '%'
+            OR u.name ILIKE '%' || $5::text || '%'
         )
     GROUP BY
         c.owner_id,

coderd/database/queries/chats.sql

@@ -725,6 +725,7 @@ WITH chat_cost_users AS (
         AND (
             @username::text = ''
             OR u.username ILIKE '%' || @username::text || '%'
+            OR u.name ILIKE '%' || @username::text || '%'
         )
     GROUP BY
         c.owner_id,