fix: filter empty messages in toModelMessage

[Tarquinen]

Summary

• Filter out messages with empty parts arrays before sending to provider API

Problem

Related to #2655

Messages with only ignored: true text parts pass the initial parts.length === 0 check but end up with empty content after filtering, causing provider API errors:

messages.X: all messages must have non-empty content except for the optional final assistant message

Solution

Filter messages with empty parts after processing, before convertToModelMessages().