fix(dom): match hidden code styles case-insensitively (#5640)

## Summary

- normalize casing and whitespace before applying the existing `display:
none` filter for `<code>` elements
- prevent hidden code payloads from entering clean Markdown when inline
CSS uses valid case or whitespace variants
- add real-browser regression coverage for lowercase, mixed-case,
tab-separated, and `!important` declarations

Fixes #5637

## Testing

- `uv run pytest tests/ci/test_html_serializer_style.py -q` — 4 passed
- `uv run pytest tests/ci/test_markdown_extractor.py
tests/ci/test_markdown_chunking.py -q` — 38 passed
- `uv run pre-commit run --files
browser_use/dom/serializer/html_serializer.py
tests/ci/test_html_serializer_style.py` — all hooks passed

Each regression case verifies that Chrome computes the element as
`display: none` and that `extract_clean_markdown` omits the hidden
payload while preserving visible content.

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes hidden code payloads from leaking into clean Markdown when inline
`style` uses valid case or whitespace variants like `Display: None` or
`DISPLAY:\tNONE`. Fixes #5637.

- Normalizes style casing and whitespace before applying the existing
`display: none` filter.
- Adds regression tests for lower-case, mixed-case, tab-separated, and
`!important` declarations.

<sup>Written for commit c9b3e5507a.
Summary will update on new commits.</sup>

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browser_use/dom/serializer/html_serializer.py

@@ -74,7 +74,8 @@ class HTMLSerializer:
 			if tag_name == 'code' and node.attributes:
 				style = node.attributes.get('style', '')
 				# Check if element is hidden (display:none) - likely JSON data
-				if 'display:none' in style.replace(' ', '') or 'display: none' in style:
+				normalized_style = ''.join(style.lower().split())
+				if 'display:none' in normalized_style:
 					return ''
 				# Also check for bpr-guid IDs (LinkedIn's JSON data pattern)
 				element_id = node.attributes.get('id', '')

tests/ci/test_html_serializer_style.py

@@ -0,0 +1,28 @@
+from urllib.parse import quote
+
+import pytest
+
+from browser_use.dom.markdown_extractor import extract_clean_markdown
+
+
+@pytest.mark.parametrize(
+	'style',
+	[
+		'display: none',
+		'Display: None',
+		'DISPLAY:\tNONE',
+		'color: red; display : none !important',
+	],
+)
+async def test_hidden_code_inline_style_is_case_insensitive(browser_session, style: str):
+	html = f'<main>visible control</main><code id="snippet" style="{style}">hidden state payload</code>'
+	await browser_session.navigate_to('data:text/html,' + quote(html))
+
+	page = await browser_session.get_current_page()
+	assert page is not None
+	computed_display = await page.evaluate("() => getComputedStyle(document.querySelector('#snippet')).display")
+	markdown, _ = await extract_clean_markdown(browser_session=browser_session)
+
+	assert computed_display == 'none'
+	assert 'visible control' in markdown
+	assert 'hidden state payload' not in markdown