fix: allow empty content in CallToolResult

[anishathalye]

Per the MCP spec [1] and the TypeScript schema [2], CallToolResult.content is typed as ContentBlock[], so it is a required array with no minimum length constraint.

MCP server libraries use such a representation in practice: for example, FastMCP returns responses with no structuredContent and an empty content array when tools return None.

---

Motivation and Context

See above.

How Has This Been Tested?

Updated unit tests, included in this PR.

Breaking Changes

This is more permissive, so I don't think this should break anything.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

N/A.

[jokemanfire]

In spec , the office's description is Tool results may contain [structured](https://modelcontextprotocol.io/specification/2025-11-25/server/tools#structured-content) or unstructured content.
Looks like the content could be empty.

[jokemanfire]

fix the ci please.

[anishathalye]

CI fixed. The one in this repo is awaiting maintainer approval: https://github.com/modelcontextprotocol/rust-sdk/actions/runs/22334296705

$ npx commitlint --from 0854700 --to 9847bde --verbose
⧗   input: fix: allow empty content in CallToolResult

Per the MCP spec [1] and the TypeScript schema [2],
`CallToolResult.content` is typed as `ContentBlock[]`, so it is a
required array with no minimum length constraint.

MCP server libraries use such a representation in practice: for example,
FastMCP returns responses with no `structuredContent` and an empty
`content` array when tools return `None`.

[1]: https://modelcontextprotocol.io/specification/2025-11-25/server/tools
[2]: https://github.com/modelcontextprotocol/specification/blob/main/schema/2025-11-25/schema.ts
✔   found 0 problems, 0 warnings

$ cargo +nightly fmt --all -- --check; echo $?
0