fix: serialize structured (dict/list) tool return values as JSON

[HumphreySun98]

Why are these changes needed?

BaseTool.return_value_as_string JSON-encodes Pydantic model return values, but
dict and list return values fall through to str(value), producing a Python
repr rather than valid JSON:

def my_tool() -> dict:
    return {"status": "success", "items": ["a", "b"]}

# return_value_as_string(...) currently yields:
#   {'status': 'success', 'items': ['a', 'b']}   <- single quotes, not valid JSON

As a result, tools returning structured data do not produce valid JSON in the
message history, and downstream agents/consumers must parse Python-repr strings
to recover the data (per #7867).

This PR serializes dict and list return values with json.dumps (falling
back to str() for values that are not JSON-serializable):

#   {"status": "success", "items": ["a", "b"]}   <- valid JSON

This is intentionally the narrow, compatibility-preserving slice discussed in the
issue: the public content: str text projection is kept unchanged (no
message-schema change, so provider adapters, serializers, summary formatting,
termination conditions, and user code that expects text are unaffected). Scalars
and other types keep their existing str() behavior; only dict/list (the
structured types the issue is about) now emit valid JSON. Pydantic returns were
already JSON-encoded and are unchanged.

Related issue number

Refs #7867 (addresses the "structured outputs are not valid JSON" part; does not
change content from str to a structured type).

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/ (none required for this change).
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed (ran ruff, mypy, and pytest tests/test_tools.py locally).