Cherry-pick of the squash commit of #44307, which serves gpt-5.6+ chat
completions on the native Bedrock /openai/v1/chat/completions route by
default and adds the chat_completions/ opt-in for gpt-oss and grok.
Conflict resolution: tests/unit/llms/bedrock/test_bedrock_common_utils.py
kept the line's file plus the one test the pick adds, since rc/1.105.0
does not carry #44112's neighbouring tests. The pick's
bedrock_route_for_request builds a MappingProxyType, whose import #44112
brought to main, so the import is added to litellm/llms/bedrock/common_utils.py
here.
(cherry picked from commit 233db9f285)
