Executable credential timeout_millis is treated as seconds

## Summary

External-account executable configuration names and validates `timeout_millis` in milliseconds, but `PluggableCredentials#subprocess_with_timeout` passes the value directly to `Timeout.timeout`, whose argument is seconds. A configured `30000` therefore waits 30,000 seconds rather than 30 seconds.

## Proposed correction

Convert milliseconds with `timeout_millis / 1000.0` at the Timeout boundary. Candidate: `6635ab3114ddc9307cb9719b40e4ac83cb389fef` on 1.17.4/current `main` (`01431c9ecd59e1efc5d262694971ded925044fdc`).

The focused clock/timeout model observes 30.0 seconds instead of 30000. 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.