Port from earendil-works/pi#9300 fix (acaa253cc): a plugin registering a
tool whose schema["parameters"] is not a dict (a list, string, etc.)
previously registered fine and the malformed schema was serialized into
every provider request, 400-ing turns far from the offending plugin.
Live probe on main confirmed the bad schema flows into _fn_def() and the
OpenAI wire unchanged.
Fail at registry.register() with the tool name in the error instead. The
plugin loader already catches registration exceptions and marks the
plugin errored, so a broken plugin degrades gracefully rather than
breaking every session. Schemas that omit "parameters" stay valid
(no-argument tools); MCP tools are unaffected (their schemas pass
through _normalize_mcp_input_schema first, which always returns a dict).
23af232837