fix: add UnsupportedCapabilityError to handle capabilities custom exception

[matteo-gobbo]

Replace generic Error with UnsupportedCapabilityError when capabilities are not supported, allowing applications to distinguish capability errors from actual failures.

Motivation and Context

It fixes the issue #430

How Has This Been Tested?

Breaking Changes

There are no breaking changes.

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

Added UnsupportedCapabilityError class in packages/core/src/types/types.ts.
I assigned to it an error code of -32043 following what's been done before, but it should be checked if it's correct.

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Latest commit: 78bb55b

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commit: 78bb55b