_strip_images_from_messages() deleted any non-tool message whose content
became empty after image removal. An assistant message whose content was
entirely images but which carried tool_calls was therefore dropped,
orphaning its paired tool responses — providers reject the next request
with unmatched tool_call_id errors (HTTP 400). Replace such messages
with the plaintext placeholder instead, exactly like tool-role messages.
Adds a regression test covering the assistant + tool_calls +
image-only-content case.
Closes#40463
cb8027afed