Better AXTree / bid marking

[gasse]

• more robust remove_redundant_static_text in flatten_axtree_to_str()
• give bids to AXTree nodes with role="generic" (html divs and spans), by using the aria-description attribute as a fallback for bid injection (using aria-roledescription does not work for AXTree nodes with role="generic")
• inject bids to all HTML elements, even non-standard ones

Reproducibility study:
gpt4o-mini agent

• on miniwob 125 tasks x 5 seeds
• on workarena.l1 33 tasks x 10 seeds

BrowserGym 0.4.2 (before change):

• miniwob 0.544
• miniwob with only AXTree 0.379
• workarena 0.221
• workarena 0.245
• workarena 0.236
• workarena 0.233

BrowserGym 0.5.0.dev0 (after change):

• miniwob 0.579 -> gain as expected (can interact with div and span)
• miniwob with only AXTree 0.365 -> within standard error?
• workarena 0.191 -> within standard error?
• workarena 0.218 -> within standard error?
• workarena 0.230 -> within standard error?
• workarena 0.233 -> within standard error?