feat(router): support runtime disabling of tools

[lutz-grex]

Add methods to disable/enable tools at runtime.
Disabled tools are hidden from listing, lookup,
and execution, including in composed routers.

Closes #477

Motivation and Context

Users of the #[tool] + #[tool_handler] macro system had no way to disable specific tools at runtime. The only workaround was manually implementing ServerHandler, losing all macro convenience, or using remove_route which permanently destroys the tool.
This change adds reversible disable/enable support directly to ToolRouter, composing naturally with the existing macro workflow:

let mut router = Self::tool_router();
router.disable_route("dangerous_tool");
// later...
router.enable_route("dangerous_tool");

How Has This Been Tested?

• 9 new unit tests in test_tool_routers.rs covering:
  disable, enable, builder pattern, merge preservation,
  remove cleanup, pre-disable before add, iterator
  filtering, and full invisibility across all query methods
• All 261 existing tests pass (cargo test -p rmcp)
• Before/after validation: confirmed disable_route and
  enable_route do not compile on the baseline and work
  correctly after the change
• cargo clippy --all-targets --all-features clean
• cargo +nightly fmt --all clean

Breaking Changes

has_route now returns false for disabled tools (previously
it only checked map membership). Code relying on has_route
to test structural presence regardless of enabled state should
use is_disabled instead. The ToolRouter struct is
#[non_exhaustive], so adding the private disabled field is
not a breaking change.

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

Three files changed:

• crates/rmcp/src/handler/server/router/tool.rs — core
  feature: disabled HashSet field, disable/enable/is_disabled
  /with_disabled methods, updated list_all/call/get/has_route
  /merge/remove_route/IntoIterator
• crates/rmcp/src/handler/server/router.rs — one-line fix:
  transparent_when_not_found dispatch now checks is_disabled
  so disabled tools cannot leak through to the inner service
• crates/rmcp/tests/test_tool_routers.rs — 9 new tests