Duplicating a Signet client drops its configured connection

## Summary

`Signet::OAuth2::Client#duplicate` copies credential options, but calls `configure_connection` only with newly supplied overrides. A connection previously configured on the source client is silently replaced by defaults when no connection override is given.

## Proposed correction

Seed duplication with `default_connection: @connection_info`, then merge explicit duplication options so caller overrides still win. Candidate: `4baf55d76e8bb1a54164b94c3779f87ad86185c2` on 1.17.4/current `main` (`01431c9ecd59e1efc5d262694971ded925044fdc`).

The focused model covers preservation, independence and explicit override behavior. The cumulative suite passes 659 RSpec examples (20 expected pending), 103 Minitest runs /201 assertions and four integration runs /41 assertions, plus RuboCop, syntax, YARD and package/install checks.

I used an AI coding assistant during source review and verification. I am opening an issue first because repository policy requires tests, a changelog and a Google CLA for pull requests; this audit cannot alter repository tests and no CLA was signed as part of it.