refactor: unify IntoCallToolResult Result impls

[DaleSeo]

Closes #440.

Motivation and Context

This PR replaces the three separate Result impls and one Result<Json<T>, E> impl with a single unified impl<T: IntoCallToolResult, E: IntoCallToolResult> IntoCallToolResult for Result<T, E>. It also adds direct impls for CallToolResult and ErrorData so they can appear in either position of a Result. The Err branch forces is_error = Some(true) on any Ok(CallToolResult) produced by the error side, which maintains backward compatibility and prevents errors from accidentally producing success results.

How Has This Been Tested?

All existing tests pass.

Breaking Changes

No. All previously valid return types continue to compile and behave identically. The change only widens the set of accepted error types.

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