Remove shared ctx from doctest namespace, use inline SessionContext

Avoid shared SessionContext state across doctests by having each
docstring example create its own ctx instance, matching the pattern
used throughout the rest of the codebase.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timsaucer

conftest.py

@@ -34,4 +34,3 @@ def _doctest_namespace(doctest_namespace: dict) -> None:
     doctest_namespace["col"] = col
     doctest_namespace["lit"] = lit
     doctest_namespace["F"] = F
-    doctest_namespace["ctx"] = dfn.SessionContext()

python/datafusion/context.py

@@ -911,6 +911,7 @@ def register_batch(self, name: str, batch: pa.RecordBatch) -> None:
             batch: Record batch to register as a table.
 
         Examples:
+            >>> ctx = dfn.SessionContext()
             >>> batch = pa.RecordBatch.from_pydict({"a": [1, 2, 3]})
             >>> ctx.register_batch("batch_tbl", batch)
             >>> ctx.sql("SELECT * FROM batch_tbl").collect()[0].column(0)
@@ -1151,6 +1152,7 @@ def register_arrow(
 
         Examples:
             >>> import tempfile, os
+            >>> ctx = dfn.SessionContext()
             >>> table = pa.table({"x": [10, 20, 30]})
             >>> with tempfile.TemporaryDirectory() as tmpdir:
             ...     path = os.path.join(tmpdir, "data.arrow")
@@ -1488,6 +1490,7 @@ def read_arrow(
 
         Examples:
             >>> import tempfile, os
+            >>> ctx = dfn.SessionContext()
             >>> table = pa.table({"a": [1, 2, 3]})
             >>> with tempfile.TemporaryDirectory() as tmpdir:
             ...     path = os.path.join(tmpdir, "data.arrow")