navigate_sub_pages mock _handle_event to let userspace handlers work

[evnchn]

Motivation

So in ui.navigate.to we kinda "cheat" and call await client.sub_pages_router._handle_navigate(path) directly.

While it works to navigate the page, userspace handlers (ui.on('sub_pages_navigate', ...)) do not work, as seen in #5487

Implementation

Don't cheat I guess? Go through the event listener system.

• We go through the event listener(s) and invoke them with mocked dictionaries.
• Userspace handlers work, of course, as of this change.
• While, client.sub_pages_router._handle_navigate is still being ran because of the following code.

nicegui/nicegui/sub_pages_router.py

Lines 18 to 22 in 7ff456b

	class SubPagesRouter:
	def __init__(self, request: Request | None) -> None:
	on('sub_pages_open', lambda event: self._handle_open(event.args))
	on('sub_pages_navigate', lambda event: self._handle_navigate(event.args))

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (c0547ef).
• Documentation has been added (or is not necessary).

[falkoschindler]

@evnchn I'm curious what @rodja thinks about this PR.
But either way, we should probably add a pytest that fails on main and passes with this PR.

[rodja]

I like it!

[rodja]

run_javascript(f'emitEvent("sub_pages_navigate", "{path}");') might be simpler, but then we will need a roundtrip to the client.

[evnchn]

I did think of that (my initial instinct), but did not go with it.