Fixes#60158
MCP tools with parameter-free schemas may return truly empty objects
`{}` without a `type` field. The existing normalization handled
`{ type: "object" }` → `{ type: "object", properties: {} }` but
missed the truly empty case.
OpenAI gpt-5.4 rejects tool schemas without `type: "object"` and
`properties`, causing HTTP 400 errors:
```
Invalid schema for function 'flux-mcp__get_flux_instance':
In context=(), object schema missing properties.
```
This change catches empty schemas (no type, no properties, no unions)
before the final pass-through and converts them to the required format.
Added test case for parameter-free MCP tool schemas.
Tools with no parameters produce { type: "object" } schemas without a
properties field. The OpenAI Responses API rejects these, silently
crashing entire sessions.
Add properties: {} injection in normalizeToolParameters() and
convertTools() to ensure all object-type schemas include a properties
field.
Closes#58246
