fix(mcpserver): preserve Annotated/Field metadata for dict[str, T] return types

[anneheartrecord]

Summary

When a tool declares a dict[str, T] return type wrapped in Annotated with
Pydantic Field metadata (e.g. a description), that metadata was silently dropped
from the generated output schema.

def get_config() -> Annotated[dict[str, int], Field(description="Configuration values")]:
    return {"timeout": 30}

print(func_metadata(get_config).output_schema)
# Before: {'type': 'object', 'additionalProperties': {'type': 'integer'}, 'title': 'get_configDictOutput'}
# After:  {'type': 'object', 'additionalProperties': {'type': 'integer'}, 'title': 'get_configDictOutput', 'description': 'Configuration values'}

Root cause

In _try_create_model_and_schema, the dict[str, T] branch called
_create_dict_model(func_name, type_expr) where type_expr is the
Annotated-stripped type. The existing TODO comment on that line even flagged
the issue. The fix is to pass original_annotation instead, so
RootModel[Annotated[dict[str, T], Field(...)]] picks up the metadata.

Changes

• src/mcp/server/mcpserver/utilities/func_metadata.py: pass original_annotation
  instead of type_expr to _create_dict_model; replace TODO comment with a
  clarifying one
• tests/server/mcpserver/test_func_metadata.py: add
  test_structured_output_dict_str_preserves_annotated_metadata regression test
  covering Field(description=...) and Field(description=..., title=...)

Fixes #2935

[anneheartrecord]

Fixed the pre-commit failure — ruff-format wrapped a long return-type annotation in the test file. All other checks were already green.