fix(messages): stand-in for user turns whose content reduces to nothing

[lailson]

Problem

formatAgentMessages converts a user message with content: "" into [{type:"text", text: ""}] and emits it with no guard (the string→array normalization runs first; an empty array is dropped, but an empty string becomes an empty text part). Providers with strict payload validation — Anthropic, Bedrock, and OpenAI-compatible gateways that validate content (reproduced against Maritaca's Sabiá API) — reject that message, and with it the entire request, with a 400. Lenient providers (OpenAI, Google, xAI) accept it silently, which hides the issue until the model list changes.

The concrete shape this arrives in: history replay of a promptless send. The stored turn keeps text: "", and once its attachments are no longer re-sent during replay, nothing fills the content — so every subsequent turn of that conversation fails on strict providers. Verified end-to-end on a LibreChat deployment using a Maritaca endpoint: byte-for-byte 400 messages: Validation error: message content cannot be empty.

Change

For user turns whose content reduces to nothing visible (null/undefined, "", [], or an array whose parts are all text parts with no visible text), substitute a single '(no text)' text part before formatMessage.

• Dropping the turn is not an alternative: adjacent turns would collapse into consecutive same-role messages that the same strict providers also reject — so the turn keeps its place with the smallest honest stand-in.
• Vision turns are untouched: any non-text part (image/file) makes a turn usable by definition.
• Assistant messages are out of scope.

Tests

Four new cases in formatAgentMessages.test.ts: empty string → stand-in; array of only-empty text parts → stand-in; vision turn with image part → untouched; assistant empty content → untouched. Full spec file green (192 passed), tsc --noEmit clean.