Add --retry-failed-tests-delay option to MTP retry package

[Copilot]

The retry extension had no way to introduce a wait between retry attempts, making it less useful for tests interacting with external systems (message queues, databases, etc.) where a cooldown period reduces flakiness.

Changes

• New CLI option --retry-failed-tests-delay: accepts a timespan value using the same unit-aware format as --hangdump-timeout (e.g. 200, 1s, 2.5m, 1h; default unit is milliseconds)
• RetryCommandLineOptionsProvider: registers the option, validates it requires --retry-failed-tests, and validates the argument via TimeSpanParser.TryParse
• RetryOrchestrator: strips the delay arg from arguments forwarded to child processes; applies Task.Delay between attempts (not after the final attempt)
• Resources: added description and invalid-argument error strings
• Tests: coverage for valid/invalid timespan values and the dependency-on-retry-option validation

Usage

dotnet test -- --retry-failed-tests 3 --retry-failed-tests-delay 2s

[Youssef1313]

Is this:

• adding the delay even when we are on the last attempt?
• Also delaying moving test results to the correct attempt results directory?

I think it's more clear to add the delay just before Process.Start exactly, with a condition that it's not the first attempt.

Overall, I don't think a single request for the feature is worth adding it, but up to you if you think it's useful enough to add it.