Add tests for config pass-through and seed-host edge cases

[m-szymon]

Current tests check config API surface and a few specific config behaviors, but do not cover value pass-through broadly or seed-host boundary cases.

Include:

• builder/config pass-through correctness for forwarded DynamoDB options such as timeout, retry config, and region
• invalid seed host handling for malformed or unusual values
• duplicate seed host handling during discovery and refresh

Expected outcome:

• forwarded config values are preserved in the resulting inner client config
• invalid seed host entries are rejected or ignored in a well-defined way
• duplicate seed hosts do not cause unstable behavior or unnecessary duplicate discovery traffic

Part of #41.