Update Snakepit.Pool.handle_call/3 to utilize resolve_pool_name_opt/2 when
determining the target pool from call options. Previously, passing a pool
name as a string would bypass internal atom mapping, leading to routing
failures when callers provided string-based identifiers.

Key changes:
- Integrated resolve_pool_name_opt/2 into the execution flow.
- Updated mix.exs and documentation to reflect the new version.
- Added a unit test to verify that string-to-pool resolution functions
  as expected during command execution.
- Updated CHANGELOG.md to document the bug fix.