[yuanhao]

Closes #369 (follow-up to #368 / #223).

Bug. An assistant turn that errored with no content — or, on Gemini/Vertex, one with only thinking, which is not replayed — was sent as content: [] / parts: []. Both APIs reject an empty message. The Anthropic builder already skipped such a turn; these three could not without putting two user turns side by side, which #368's merge now handles.

Fix. provider::merge_same_role drops any message whose key array is empty before merging, so the user turns around a dropped one merge into one. All three builders go through it; a message without the key is still kept as it is.

Tests (each fails without the fix — checked by disabling the drop):

• merge_same_role_tests::empty_messages_are_dropped_and_their_neighbours_merge — an empty model turn between two user turns, and a trailing empty one.
• bedrock::tests::an_empty_assistant_turn_is_not_sent — [user, assistant(no content, Error), user] → one user message.
• google::tests::an_empty_model_turn_is_not_sent — a thinking-only model turn is dropped and the user turns merge.

Checks: --all-features test / clippy / doc / fmt clean. Not run against a live endpoint.

🤖 Generated with Claude Code

https://claude.ai/code/session_01T7iq5hpndSiHQcnAsywKuG

[yuanhao]

Review (code-reviewer): no critical issues; traced that no tool call or tool result can become empty, so nothing gets unpaired; the dropped-turn shape is the one #223 already produces. Fixed in e6c3400: two existing test doc comments my insertion had split (bedrock.rs, google.rs), and the CHANGELOG now says empty user messages are dropped too.

[yuanhao]

Test + comment reviews addressed in 6f0268a: tests use the loop's real error shape (one empty text block) as well as no content; an empty message first and an empty user message; a Vertex builder test; and a Bedrock end-to-end test — a failed prompt then another, the second request carries one user message (fails without the fix), history keeps the errored turn. Docs widened to errored/refused/cancelled turns and unreplayable thinking on all three providers; bedrock.md now says Anthropic redacted blocks are logged at debug.