fix: Use case sensitive filter for phone_numbers (#12470)

The contact filter APIs were timing out due to the case‑insensitive
filter. There is no index for lower case phone numbers, so it would
perform a table scan, potentially examining 8 million records or more at
a time.

This change should fix the issue. 
I am changing the filter to use direct comparison without lower‑case.

**Previous:**
```sql
SELECT contacts.*
FROM contacts
WHERE contacts.account_id = $1
  AND (
    LOWER(contacts.phone_number) = '<number>'
    OR LOWER(contacts.phone_number) = '<other-number>'
  )
ORDER BY contacts.created_at DESC NULLS LAST
LIMIT $2
OFFSET $3
```

**Updated:**
```sql
SELECT contacts.*
FROM contacts
WHERE contacts.account_id = $1
  AND (
    contacts.phone_number = '<number>'
    OR contacts.phone_number = '<other-number>'
  )
ORDER BY contacts.created_at DESC NULLS LAST
LIMIT $2
OFFSET $3
```

Fixes:
https://linear.app/chatwoot/issue/CW-5582/contact-filter-timing-out

Gemfile.lock

@@ -727,7 +727,7 @@ GEM
     retriable (3.1.2)
     reverse_markdown (2.1.1)
       nokogiri
-    rexml (3.4.1)
+    rexml (3.4.4)
     rotp (6.3.0)
     rspec-core (3.13.0)
       rspec-support (~> 3.13.0)