Bedrock's Converse API multiplexes Claude, Llama, Titan, Mistral and
others through a single content-block protocol. None of those families
expose a per-message author identity in a unified way, so prefixing in
the text content block keeps multi-user attribution consistent
regardless of which underlying model the request lands on.
buildUserContent now passes msg through messageutil.ApplyUserNamePrefix
before constructing the text ContentBlockMemberText. Tool result paths
go through a separate code path (toolResultBlock) and are unaffected.
Image media handling is unchanged.
Tests:
- buildUserContent with Name set produces the prefixed text block
- buildUserContent without Name preserves byte-identical behavior
- existing image and base64 cases continue to pass
The Bedrock build is gated behind //go:build bedrock so this path is
only exercised in builds that opt in via that tag.
Refs #2702.
5b0634286c