Add interface type-checking for timeSource config in RequestDelay

[unprolix]

Summary

• Replace MustBe.object() with MustBe.instanceOf(value, IntfTimeSource)
• Validates that non-null timeSource values implement the IntfTimeSource interface
• Matches the pattern used in TokenBucket

Test plan

• Pass valid IntfTimeSource implementation, verify accepted
• Pass StdTimeSource.INSTANCE explicitly, verify accepted
• Pass plain object without required methods, verify rejected with descriptive error
• Pass null, verify StdTimeSource.INSTANCE is used (existing behavior preserved)
• Existing tests pass
• Linter passes