fix(http-client-python): make vendored streaming_base banner flavor-aware

The vendored _utils/streaming_base.py header hardcoded "vendored from
azure-core (azure.core.streaming)". For the unbranded flavor this leaked
the word "azure" into generated output, failing the unbranded
test_sensitive_word check. Reword the banner to reference
{{ code_model.core_library }}.streaming (azure.core.streaming for azure,
corehttp.streaming for unbranded), consistent with the flavor-aware
import just below it.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 51359cbf-bc53-429e-b947-8284a91d7d46

Author: l0lawrence

packages/http-client-python/generator/pygen/codegen/templates/streaming_base.py.jinja2

@@ -4,11 +4,11 @@
 {% endif %}
 # pylint: disable=line-too-long,useless-suppression,unnecessary-ellipsis
 # --------------------------------------------------------------------------
-# This file is vendored from azure-core (azure.core.streaming). It provides the
-# Stream / AsyncStream helpers (plus the JSONL / SSE decoders and event types)
-# used by generated structured-streaming operations, so the generated package
-# does not take a hard dependency on a core runtime that ships the streaming
-# helpers. Do not edit by hand.
+# This file is vendored from the core streaming runtime
+# ({{ code_model.core_library }}.streaming). It provides the Stream / AsyncStream
+# helpers (plus the JSONL / SSE decoders and event types) used by generated
+# structured-streaming operations, so the generated package does not take a hard
+# dependency on a core runtime that ships the streaming helpers. Do not edit by hand.
 # --------------------------------------------------------------------------
 import codecs
 import json