Support Secret type in Open Browser url

[JJT1an]

Fixes #1993.

Open Browser passes the url argument straight to the driver, so passing a Robot Framework Secret (as in the issue example) calls driver.get() with the Secret object instead of the actual URL. Input Text and Input Password already unwrap Secret values; this does the same for url.

The url argument is now typed str | Secret | None, the value is unwrapped before it reaches the driver, and the log messages keep the masked <secret> representation so the value is not written to logs.

How checked: python utest/run.py → 257 passed, 4 skipped. The new unit test test_open_browser_with_secret_url fails on master with Instead got: get(Secret(value=<secret>)) and passes with the fix.

This covers the url argument only. Other keywords taking string arguments may still not accept Secret; I am happy to extend the scope if you prefer a broader change.