llms/googleai: return error instead of panicking on system-only message list

[SAY-5]

Fixes #1507

generateFromMessages builds a history slice from the input messages, skipping any system message (it is routed to model.SystemInstruction). When every message has role system, the slice ends up empty and history[n-1] panics with index out of range [-1].

This guards the empty case and returns a new ErrNoMessageContent error instead of panicking, so a system-only message list (including in stream mode) fails cleanly.

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