fix(mcp): preserve $defs in McpWorkbench.list_tools() for $ref resolution

[octo-patch]

Fixes #6663

Problem

When an MCP tool's inputSchema contains $defs for reusable type definitions,
McpWorkbench.list_tools() only extracted properties, required, and
additionalProperties into the ParametersSchema, silently dropping $defs.

This means any properties that reference types via $ref: "#/$defs/SomeName" are
forwarded to the model client with unresolvable references — the LLM sees e.g.
{"$ref": "#/$defs/Name"} but without the $defs dict it cannot know the actual
type. This is particularly common with MCP servers whose tools share enums or
sub-schemas across parameters.

Solution

Include $defs from inputSchema in the outgoing ParametersSchema dict when
present. OpenAI's FunctionParameters type is Dict[str, object], so it accepts
$defs and will forward them to the API where they can be used to resolve $ref
entries in the properties. Schemas without $defs are unaffected.

The change is a two-line addition in _workbench.py plus a matching test.

Changes

• python/packages/autogen-ext/src/autogen_ext/tools/mcp/_workbench.py: preserve
  $defs when building ParametersSchema in list_tools()
• python/packages/autogen-ext/tests/tools/test_mcp_workbench_overrides.py: add
  tests for schemas with and without $defs

Testing

• Added test_mcp_workbench_list_tools_preserves_defs: verifies $defs is
  forwarded and $ref entries in properties are intact
• Added test_mcp_workbench_list_tools_without_defs: verifies unchanged behavior
  for schemas that don't use $defs