feat(macros): reject an empty #[tool_router]

[DaleSeo]

Closes #1174.

Motivation and Context

An impl annotated with #[tool_router] that had no directly visible #[tool] functions compiled without any diagnostics and produced a router with zero tools. The server advertised an empty catalogue, and the problem only showed up on the other side of the connection, when a model found no tools.

#[tool_router] now rejects an impl with no #[tool] functions instead of quietly building a router that serves nothing. Use #[tool_router(allow_empty)] to opt out.

How Has This Been Tested?

Add tests

error: `#[tool_router]` found no `#[tool]` fn in this impl block, so `Self::tool_router()` would serve no tools
       note: a `macro_rules!` invocation inside the impl block is not expanded before this
             attribute runs, so any `#[tool]` fn it generates is invisible here; let the
             `macro_rules!` emit the whole `#[tool_router] impl` instead
       note: use `#[tool_router(allow_empty)]` if an empty router is intended
  --> src/probe.rs:15:6
   |
15 | impl Probe {
   |      ^^^^^

Breaking Changes

There is no API change, but an impl block annotated with #[tool_router] no longer compiles if it doesn't contain any #[tool] functions. This is intended because an impl with no tools is almost always a mistake.

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