Fix tool result serialization when the tool returns a list

[strawgate]

Fixes #378

[Copilot]

Pull Request Overview

This PR fixes tool result serialization when a tool returns a list by ensuring that a custom serializer is correctly propagated.

• Added a test (test_call_tool_with_list_result_custom_serializer) that verifies a custom serializer is applied to list results.
• Updated _convert_to_content to pass the serializer parameter during recursive conversion of list items.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/tools/test_tool_manager.py	Added a new test to ensure custom serialization logic for lists.
src/fastmcp/tools/tool.py	Propagated the serializer in recursive calls to _convert_to_content.

[jlowin]

Good catch!