The MCP voicebox.speak tool built its GenerationRequest without a
model_size, so every agent-triggered generation fell back to the schema
default ("1.7B"). There was no way to reach the 0.6B Qwen variant (or
TADA's 1B/3B) through MCP, and callers paid a model reload whenever the
requested size differed from what was already loaded.
Thread an optional model_size through voicebox.speak and the _speak
helper into GenerationRequest, mirroring the REST /generate surface.
Omitting it passes None, which generate_speech normalizes to the engine
default, so existing callers are unaffected.
Add backend/tests/test_mcp_speak.py covering the forwarded value, the
omitted-default path, and rejection of an invalid size.
Fixes#884
