RandomDate ignores inclusive end date and throws when start == end

### Expected behavior

The ` __RandomDate` function should treat the provided end date as inclusive. When the start and end dates are the same, the function should simply return that single date (and store it in the target variable) instead of failing.

### Actual behavior

- When `startDate < endDate`, the function never returns the configured end date. It behaves as if the interval were [start, end) (exclusive upper bound).

- When `startDate == endDate`, the call to `ThreadLocalRandom.nextLong(start, end)` raises `IllegalArgumentException: bound must be greater than origin`, so the function errors out instead of returning the only valid date.

### Steps to reproduce the problem

1. Create a test plan with a Debug Sampler and a Simple Data Writer (or just watch the Debug Sampler output).
2. Add a User Parameters or a JSR223 Sampler to evaluate` ${__RandomDate(yyyy-MM-dd,2111-03-29,2111-03-30,,MY_VAR)}`.
3. Run the plan multiple times. Observe that the logged value is always `2111-03-29` and `MY_VAR` never contains `2111-03-30`.
4. Change the call to `${__RandomDate(yyyy-MM-dd,2111-03-29,2111-03-29,,MY_VAR)}` and run again. The sampler now fails with the `IllegalArgumentException` mentioned above.

### JMeter Version

6.0.0-SNAPSHOT

### Java Version

OpenJDK Runtime Environment 21.0.8 (2025-07-16)

### OS Version

Linux 6.6.87.2-microsoft-standard-WSL2 (running under Windows 11 / WSL2)