Bedrock/Gemini/Vertex: drop empty messages before merging (an errored assistant turn sends content: []) #369

Bedrock, Gemini and Vertex send a message with no content (content: [] / parts: []) in two cases. Both APIs reject an empty message:

• an assistant turn that errored and left no content (or, on Gemini, only thinking, which is not replayed);
• a user message whose only text is empty.

The Anthropic builder skips such an assistant turn (anthropic.rs ~546-553). The other three could not do the same, because dropping it would have put two user messages side by side.

#368 (for #223) merges consecutive same-role messages in those builders, so now they can drop empty messages before merging, and the user messages around a dropped turn merge into one.

Fix: in build_bedrock_body, google::build_request_body and google_vertex::build_vertex_request_body, filter out messages whose content / parts array is empty, then call merge_same_role. Add a test with an errored, content-less assistant turn between two user turns.

Found by the review of #368. This existed before that PR; the PR does not create it.