fix(bedrock): send a placeholder for empty assistant turns on Converse

[ebarkhordar]

Fixes #426.

The audit turned up one gap, and it is on the Converse side. as_converse_messages() appends each turn's content array unconditionally, so an assistant turn with no content blocks goes on the wire as content: [], which Converse rejects. _as_turn() produces exactly that turn when a response carries no content blocks, including the case where message is absent because a guardrail intervened before the model produced any. This sends the same "[empty string]" placeholder as #421 rather than dropping the turn, so user/assistant alternation stays intact.

Empty thinking blocks are already handled here. content_from_converse_block() reads reasoningContent.get("reasoningText") or {} and then .get("text", ""), so a block carrying only a signature, one with no reasoningText at all, one where it is None, and a redactedContent block all return ContentThinking(thinking="") without raising. I checked those four shapes and changed nothing there.

The other two Bedrock providers need nothing either. BedrockMessagesProvider subclasses AnthropicProvider and does not override _as_message_params, so #421 covers it, and BedrockResponsesProvider goes through the OpenAI path.

Verified at c21e2451:

• test_empty_assistant_turn_gets_a_placeholder fails without the change, with assert [] == [{'text': '[empty string]'}].
• test_empty_user_turn_is_left_alone pins the scope rather than the bug, so I checked it can fail: dropping the role check from the guard turns it red.
• Full suite on 3.10 and 3.14, ruff check chatlas clean, and pyright reports only the one pre-existing _content_image.py error that main reports too.

I could not check the rejection itself, since this machine has no AWS credentials, and botocore's service model puts no minimum on the content array, so nothing client-side complains about content: [] today. A caller who passes their own messages through kwargs still bypasses this path.

[ebarkhordar]

The three red workflows are not from this branch.

test (3.12) runs the suite green (1265 passed, 46 skipped) and then dies in make check-types on chatlas/_content_image.py:159, where pyright rejects register_heif_opener as not exported from pillow_heif. test-live fails 362 tests on Missing credentials ... OPENAI_API_KEY, which a fork PR does not get. Check provider types and pricing asks for make update-types, make update-pricing and make update-bedrock-apis to be run and committed.

This branch sits on c21e245, 0 ahead and 0 behind main, and touches only the bedrock converse provider, its test and the changelog. Those three workflows last ran on main at that same commit on 2026-09-04 and were green then.

Separately, I moved the changelog entry into a new ## [Unreleased] section. It had gone under ## [0.23.0] - 2026-09-04, which had already shipped.

[cpsievert]

Thanks!