fix: accept empty dict as valid tool_args

[thakoreh]

Problem

`validate_tool_request` rejects `{"tool_name": "x", "tool_args": {}}` because `not tool_request.get("tool_args")` evaluates `{}` as falsy.

Fix

Replace the falsy check with `not isinstance(tool_request.get("tool_args"), dict)`. Empty dict is valid, missing or non-dict is not.

Test

5 new tests covering: empty args (valid), missing args (invalid), wrong type (invalid), normal args (valid), None args (invalid).

[3clyp50]

Solved in #1466