feat: add local feature for !Send tool handler support

[DaleSeo]

Closes #728

Motivation and Context

Some MCP server use cases involve state that isn't Send. For example, this includes holding a raw pointer, an Rc, or a thread-local resource. The current service stack requires Send + Sync everywhere, which completely rules these out. This PR introduces a local Cargo feature that relaxes all Send and Sync bounds across the service stack. This change allows servers with !Send handlers to run on a single-threaded runtime using tokio::task::LocalSet and spawn_local.

How Has This Been Tested?

The full test suite passes in both modes:

• cargo test --features server,client (default, no behavior change)
• cargo test --features server,client,local (new path).

Breaking Changes

None. Without the local feature the effective trait bounds are identical to before — MaybeSend resolves to Send + Sync and MaybeSendFuture resolves to Send via blanket impls. The feature is strictly opt-in.

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

[alexhancock]

It LGTM as an opt-in thing.

Perhaps good to get @Stranmor to look as well to double confirm it addresses the need?

[Stranmor]

This PR fully addresses the use case I described in #728. The global local feature approach works well for us — MCP servers are typically single-threaded, so relaxing Send+Sync bounds across the board is a practical trade-off. Appreciate the clean implementation with MaybeSend/MaybeSyncSend traits. Thanks @DaleSeo for picking this up!