Improve MCP server validation and preview action error handling

[SCPgamerscp]

Motivation

• Fix a couple of robustness issues in the Python MCP server that cause unpredictable behavior when calling the YMM4 HTTP API.
• Prevent invalid inputs (e.g. zero/negative speech speed) from producing incorrect timing calculations or runtime errors.
• Provide clearer failure behavior for unsupported preview actions and make returned script summaries more readable.

Description

• Modified ymm4_post signature to async def ymm4_post(path: str, body: dict | None = None) and construct a local payload to avoid a mutable default argument and ensure safe POST bodies.
• Added validation in add_script to raise ValueError when chars_per_sec <= 0 to avoid invalid length calculations.
• Changed script result summary logic so preview text only appends ... when the original text is longer than 20 characters.
• Added an explicit case _ branch in dispatch_preview to raise a ValueError for unknown preview action values so unsupported actions fail fast and clearly.
• File modified: mcp-server/server.py.

Testing

• Ran python -m py_compile mcp-server/server.py and the file compiled successfully with no syntax errors.

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Codex Task