feat!: complete deprecation and cleanup of multimodal blob APIs (#16618)

This PR deprecates and cleans up experimental Multimodal Blob APIs in
BigQuery DataFrames to focus on core supported features.

Key Changes:

- Removed public APIs: from_glob_path and read_gbq_object_table were
removed from module exports and Session class.

- Exposed internal API: Added _from_glob_path as an internal API in the
module to support existing notebooks.


Fixes #<478952827> 🦕

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: shuoweil

packages/bigframes/bigframes/bigquery/_operations/ai.py

@@ -25,6 +25,7 @@
 
 from bigframes import dataframe, dtypes, series, session
 from bigframes import pandas as bpd
+from bigframes.bigquery._operations import obj as bq_obj
 from bigframes.bigquery._operations import utils as bq_utils
 from bigframes.core import convert
 from bigframes.core.compile.sqlglot import sql as sg_sql
@@ -1183,7 +1184,7 @@ def _separate_context_and_series(
     if isinstance(prompt, series.Series):
         if prompt.dtype == dtypes.OBJ_REF_DTYPE:
             # Multi-model support
-            return [None], [prompt.blob.read_url()]
+            return [None], [bq_obj.get_access_url(prompt, mode="R")]
         return [None], [prompt]
 
     prompt_context: List[str | None] = []
@@ -1220,7 +1221,7 @@ def _convert_series(
 
     if result.dtype == dtypes.OBJ_REF_DTYPE:
         # Support multimodal
-        return result.blob.read_url()
+        return bq_obj.get_access_url(result, mode="R")
     return result