Guard empty Bedrock Converse tool results against empty text blocks

[MaxFreedomPollard]

Fixes #55092

convert_messages_to_converse built the toolResult text block directly from the raw tool content with no empty-content guard:

result_content = content if isinstance(content, str) else json.dumps(content)
tool_result_block = {"toolResult": {"toolUseId": ..., "content": [{"text": result_content}]}}

When a tool returns an empty string, this is a {"text": ""} block. Bedrock Converse rejects empty text blocks (ValidationException: ... text content blocks must be non-empty). Because the block is part of the persisted history, every replay and retry fails identically, wedging the conversation.

_convert_content_to_converse already guards this for user and assistant content (issue #9486), but the tool-result branch does not go through that helper and was left unguarded.

Change

• Substitute the same non-empty placeholder the user/assistant path uses when the serialized tool result is empty or whitespace-only.

Test

test_empty_tool_result_gets_placeholder converts a tool message with empty content and asserts the resulting toolResult text block is non-empty. It fails on the current code (the block is {"text": ""}) and passes with this change. The rest of tests/agent/test_bedrock_adapter.py continues to pass.

$ python -m pytest tests/agent/test_bedrock_adapter.py -q
133 passed

[Morad37]

Clean fix. Empty text blocks are a known Bedrock Converse constraint -- the existing _convert_content_to_converse guard (issue #9486) covers user/assistant content but missed the tool result path. This closes the gap.

One note: result_content = " " (space) is what the existing user/assistant guard uses too -- consistent and avoids the 400 without changing visible behavior since whitespace gets dropped downstream anyway.

[teknium1]

Thanks for the focused Bedrock fix. The premise remains valid on current main: agent/bedrock_adapter.py:586-590 serializes an empty string unchanged into a Converse toolResult text block. The proposed guard matches the established whitespace-placeholder behavior in _convert_content_to_converse at agent/bedrock_adapter.py:504-507.

The regression test is targeted and exercises the affected converter path. The current-main file blobs equal the PR diff preimages, so this should be salvageable as a clean cherry-pick.

Automated hermes-sweeper review.