fix(http_pool): add retry logging

[capy-ai]

This PR adds logging to the HTTP connection pool retry logic in src/http_pool.nim, which previously swallowed BadClientError and ProtocolError exceptions silently. The retry behavior remains unchanged — only visibility is improved.

Changes

• src/http_pool.nim: Log retry attempts with exception name and message using [http_pool] prefix
• src/http_pool.nim: Wrap retry in nested try/except to log and re-raise if retry also fails