feat: add display.render_mode to control DataFrame/Series visualization (#2413)

This PR introduces the display.render_mode configuration option,
providing a clearer and more flexible way to control how BigFrames
objects are visualized in notebooks and other interactive environments.

  Key Changes:
* New Configuration Option: Added bpd.options.display.render_mode which
supports three modes:
       * html (Default): Standard HTML table rendering.
* plaintext: Forces plain text output by removing the HTML component
from the mimebundle.
* anywidget: Enables the interactive anywidget-based table component.
* Migration Path: Maintained backward compatibility for
display.repr_mode = "anywidget". The rendering logic now checks both the
new render_mode and the legacy repr_mode flags.
* Notebook & Test Updates: Updated existing notebooks and system tests
to prefer the new render_mode option while verifying that existing
behaviors remain intact.
* New Unit Tests: Added tests/unit/display/test_render_mode.py to
specifically verify the mimebundle selection logic and priority across
different configuration states.

While repr_mode was previously used to toggle the interactive widget,
"representation mode" is a broad term. render_mode specifically
addresses the format of the output (HTML vs. Text vs. Widget), allowing
for better support for text-only environments and a more intuitive API
for users.

Also verified at colab notebook:
[screen/AHNz4o5Mhb9UHrh](https://screenshot.googleplex.com/AHNz4o5Mhb9UHrh)

Fixes #<479282023> 🦕

Author: shuoweil

bigframes/display/html.py

@@ -361,7 +361,7 @@ def repr_mimebundle(
     if opts.repr_mode == "deferred":
         return repr_mimebundle_deferred(obj)
 
-    if opts.repr_mode == "anywidget":
+    if opts.render_mode == "anywidget" or opts.repr_mode == "anywidget":
         try:
             with bigframes.option_context("display.progress_bar", None):
                 with warnings.catch_warnings():
@@ -380,4 +380,8 @@ def repr_mimebundle(
                 f"Falling back to static HTML. Error: {traceback.format_exc()}"
             )
 
-    return repr_mimebundle_head(obj)
+    bundle = repr_mimebundle_head(obj)
+    if opts.render_mode == "plaintext":
+        bundle.pop("text/html", None)
+
+    return bundle