fix(bedrock): replace whitespace-only ' ' placeholder with '(empty message)' in Converse adapter

[HeLLGURD]

Bug

�gent/bedrock_adapter.py uses a single space ({"text": " "}) as a placeholder for empty/missing message content and for the synthetic user-padding turns that enforce Converse's strict user/assistant alternation requirement. AWS Bedrock's Converse/ConverseStream API - when fronting Anthropic Claude models - rejects whitespace-only text blocks:

ValidationException: text content blocks must contain non-whitespace text

This is the sequel to #9486. That fix replaced empty strings with " ", which passes the non-empty check but fails the non-whitespace check introduced subsequently by the Converse validation layer. The result is a hard, non-retryable failure that fires every time a persisted session is resumed whose earliest stored message is an assistant turn (triggering the leading-user padding path).

Impact

Every �edrock / �edrock_converse request that hits any of the following paths produces an unrecoverable ValidationException:

• Resuming a session whose history starts with an assistant message (leading-user pad)
• None or empty-string message content (_convert_content_to_converse)
• Whitespace-only text parts ("\n", "\t", etc.) passing the bare-truthiness gate
• An assistant message with no text and no tool calls (empty-assistant fallback)

Fix

Replace all seven {"text": " "} placeholder occurrences in _convert_content_to_converse() and convert_messages_to_converse() with {"text": "(empty message)"}, mirroring the already-correct handling in �gent/anthropic_adapter.py::_convert_user_message. Also tighten the truthiness gate on individual text parts from ext if text to ext if text.strip() so that genuinely whitespace-only text blocks (e.g. "\n") are also caught and replaced with the non-whitespace placeholder.

No behaviour change for any request that does not hit these paths.

Closes #39829

[teknium1]

Thanks for isolating the six literal placeholder sites; the reported assistant-first and empty-content failure is still present on current main at agent/bedrock_adapter.py:505-507, :629, and :655-659.

Problems

• The change does not yet enforce the stated non-whitespace contract for all Converse text blocks. Current paths still pass raw string list items unchanged at agent/bedrock_adapter.py:512, system list text at :578-580, and tool-result content at :586-590. These paths are outside this diff.
• The PR changes no tests. Current TestEmptyTextBlockFix instead asserts the obsolete space placeholder at tests/agent/test_bedrock_adapter.py:1307-1323.

Suggested changes

• Add conversion-level regressions asserting no emitted text block (including nested toolResult.content and system blocks) is whitespace-only.
• Extend the same sanitizer/policy to the remaining paths while preserving tool-result pairing.

Automated hermes-sweeper review.

[teknium1]

This handles dict-shaped text parts, but _convert_content_to_converse() still appends raw string list entries unchanged on current main (agent/bedrock_adapter.py:512). Please sanitize a content array such as ["\n"] too, and cover it with a regression test.