fix: GeckodriverElement W3C serialization for connect-existing mode

[freema]

Summary

• GeckodriverElement serialized as {} when passed through JSON.stringify(), because it only had private fields and no toJSON() method
• Added toJSON() returning the W3C element reference format ({"element-6066-11e4-a52e-4f735466cecf": id})
• This fixes all UID-based tools that pass elements via executeScript() or the Actions API in --connect-existing mode

Affected tools

Tool	Issue	Root cause
hover_by_uid	Broken	actions().move({ origin: el }) serialization
drag_by_uid_to_uid	Broken	executeScript(..., fromEl, toEl)
screenshot_by_uid	Partial	scrollIntoView(arguments[0])
upload_file_by_uid	Partial	executeScript(..., el)
evaluate_script	Broken (with UID args)	executeScript(..., resolvedArgs)

Tools using direct HTTP calls (click_by_uid, fill_by_uid) were not affected.

Test plan

• 3 new unit tests verifying W3C serialization via findElement, executeScript, and actions().move()
• 424/424 tests pass

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2027635

[juliandescottes]

Thanks for the fix!

I don't know the connect-existing setup well enough, but now that the issue related to BiDi was fixed, do you think we could use a regular driver instance?

In general, it would be great if connecting to an existing instance was using the same setup to avoid bugs and inconsistencies.