fix(mcpserver): serialize empty list and tuple returns into TextContent

[Maazkorejo]

Description

Fixes #3307

When a @mcp.tool() function returns an empty list [] or tuple (), _convert_to_content previously collapsed the return value into an empty list of content blocks (content: []). This caused downstream clients (such as Claude Desktop, LangGraph, and IDE integrations) to treat valid empty results as errored executions due to zero content blocks.

Changes Made

• Updated _convert_to_content in src/mcp/server/mcpserver/utilities/func_metadata.py to return [TextContent(type="text", text="[]")] for empty lists (and "()" for empty tuples).
• Added unit test in tests/server/mcpserver/test_func_metadata.py.
• Added end-to-end client/server interaction test in tests/interaction/mcpserver/test_tools.py.

Checklist

• Linked existing issue Streamable HTTP clean EOF reconnects can exceed the request retry budget #3307.
• Unit and interaction tests added.

[github-actions]

Thanks for the contribution. This repository only keeps pull requests open when they're linked to an issue that a maintainer has assigned to the author — CONTRIBUTING.md explains why and how we work. This PR has been closed for now because you aren't currently assigned to #3307.

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