Prevent sending thinking blocks with no signature with ChatBedrockConverse

[tahouse]

Fixes #401

Claude 3.7 still works because signatures are always returned with those thinking blocks. DeepSeek thinking blocks do not contain signatures and the ValidationError indicates a signature must be included with thinking blocks.

According to Converse API docs:

"Within the reasoningText field, the text fields describes the reasoning. The signature field is a hash of all the messages in the conversation and is a safeguard against tampering of the reasoning used by the model. You must include the signature and all previous messages in subsequent Converse requests. If any of the messages are changed, the response throws an error."

https://docs.aws.amazon.com/bedrock/latest/userguide/conversation-inference-call.html

@3coins I posted a separate PR #407 to make reviewing easier. Happy to combine if preferred.

[tahouse]

Fixed the failing test, make lint_tests passes now with the added annotation. I ran make format, but it doesn't seem to fix all the linting issues found by make lint -- mainly, long line lengths are not being corrected. I fixed chat_models/bedrock_converse.py and test_bedrock_converse.py with manually breaking up lines, however 30 more manual edits will be needed to fix the other files.

[tahouse]

Tested and passing make lint_tests for Python 3.9 now as well

[michaelnchin]

Integration CI run on this branch looks clean. One unrelated test for ChatBedrock failed due to a test account permissions issue - I verified that this test is passing locally.

[michaelnchin]

Thanks @tahouse !