* fix(bedrock-converse): drop blank-text fallback for empty thinking blocks
Claude Code with extended thinking replays prior assistant turns that
include an empty thinking block (`thinking=""`, `signature=""`) alongside
tool_use blocks. The unsigned-reasoning fallback in
`add_thinking_blocks_to_assistant_content` was emitting
`BedrockContentBlock(text="")`, which Bedrock Converse rejects with:
"The text field in the ContentBlock object at messages.X.content.0
is blank."
Guard the fallback with a strip() check, matching the existing
empty-text guards elsewhere in `_bedrock_converse_messages_pt`.
* style: remove unneeded comments