Bedrock Converse rejects text content blocks that are empty OR
whitespace-only (ValidationException: "text content blocks must
contain non-whitespace text"). The prior fix attempt substituted a
single space (" ") for missing content -- but a lone space IS
whitespace, so it was rejected by the exact same validation rule it
was meant to satisfy. This caused a deterministic, unrecoverable
retry-loop failure once any blank/whitespace assistant, tool, or user
turn entered history (most commonly via context-compaction rewriting
a turn to a blank string).
Adds _safe_text()/_EMPTY_TEXT_PLACEHOLDER ("(empty)") and applies it
everywhere a blank text block could reach the wire: user/assistant
content conversion, tool results, the assistant-empty-turn fallback,
and the first/last-message user-alternation padding. System-prompt
blocks are the one exception: blank parts are dropped entirely rather
than placeholder-filled, since a system prompt block should never
carry meaningless placeholder text.
Adds tests/agent/test_bedrock_empty_text_blocks.py (11 tests, was
already present uncommitted -- codifies the exact failing history
from issue #9486 and asserts no blank block ever reaches Bedrock).
Verified against the actual failed request dump from this session
(27-message payload) -- replaying it through the fixed converter now
produces zero blank/whitespace-only blocks.
