Add DOM-guarded CSS/JS injection to DataFrameHtmlFormatter to prevent duplicate style/script inserts

[kosiew]

Description:

Currently, DataFrameHtmlFormatter tracks a _styles_loaded class flag in Python to avoid re-injecting styles and scripts across notebook cells. However, this approach:

• Doesn’t survive module reloads or redefinitions
• Can conflict if multiple notebooks share the same kernel
• Still requires manual state resets in tests

We should move the “only-once” guard into the browser DOM/JS layer so that every call to format_html can blindly emit the header scripts/styles, but the browser will insert them only once per page.

Proposed Change:

1. Remove the _styles_loaded class variable and related Python guard logic.
2. Always call _build_html_header() and _get_javascript() on each format_html.
3. Inside those methods, wrap the CSS/JS in small <script> blocks that:

• Check document.getElementById('df-styles') (or a global flag) before appending a <style> tag
• Check a window.df_formatter_js_loaded flag before defining the toggle function

example snippet:

--- a/python/datafusion/dataframe_formatter.py
+++ b/python/datafusion/dataframe_formatter.py
@@ -XXX,XX +XXX,XX @@ class DataFrameHtmlFormatter:
-    # Class var: _styles_loaded = False

     def _build_html_header(self) -> list[str]:
-        html = []
-        # Only include styles and scripts if needed
-        if include_styles:
-            html.extend(self._build_css_and_script())
-            if self.use_shared_styles:
-                DataFrameHtmlFormatter._styles_loaded = True
-        return html
+        return ["""
<script>
// Only inject CSS once per page
if (!document.getElementById('df-styles')) {
  const style = document.createElement('style');
  style.id = 'df-styles';
  style.textContent = `
    /* DataFrameHtmlFormatter CSS… */
  `;
  document.head.appendChild(style);
}
</script>
"""] + ([f"<style>{self.custom_css}</style>"] if self.custom_css else [])

     def _get_javascript(self) -> str:
-        return """
-<script> …existing JS… </script>
-"""
+        return """
<script>
// Only inject toggle logic once
if (!window.__df_formatter_js_loaded__) {
  window.__df_formatter_js_loaded__ = true;
  window.toggleDataFrameCellText = function(table_uuid, row, col) {
    /* …existing toggle logic… */
  };
}
</script>
"""

Benefits:

• Guaranteed “inject once” directly in the browser, regardless of Python reloads
• Simplifies Python state—no _styles_loaded flag to manage
• Safe in multi-notebook or testing scenarios