Drop redundant .to_s inside string interpolation

In `CallableStatement#bind_param_index` the leading-colon normalization
wrote `":#{key.to_s}"`. String interpolation already invokes `to_s`
on the embedded expression, so the explicit call is redundant.

The `key.to_s =~ /^:/` on the right-hand side is intentionally kept;
that one is outside the interpolation and ensures the regex match
receives a String regardless of whether `key` is a Symbol or String.

No behavior change. Full suite is green (468 examples, 0 failures,
1 pre-existing pending).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

lib/plsql/jdbc_connection.rb

@@ -189,7 +189,7 @@ def close
 
         def bind_param_index(key)
           return key if key.kind_of? Integer
-          key = ":#{key.to_s}" unless key.to_s =~ /^:/
+          key = ":#{key}" unless key.to_s =~ /^:/
           @params.index(key) + 1
         end
     end