refactor!: remove deprecated v3 APIs

[DaleSeo]

Motivation and Context

The v3 release is the appropriate breaking-change window to remove compatibility APIs that have completed their deprecation cycle. This PR removes deprecated aliases, obsolete builder helpers, and unreachable error variants whose replacements have been available since before v3. It retians protocol compatibility APIs that are still needed, including deprecations introduced for the 2026-07-28 spec.

How Has This Been Tested?

All tests are still passing. Reviewers can verify the change.

Breaking Changes

Yes. Code using removed aliases must migrate to their established replacements, such as plural *Params names, Elicit*, ResourceTemplateReference, SamplingMessageContentBlock, PrimitiveSchemaDefinition, MetaObject, and ErrorData. Callers of the removed enum builder shortcuts should use required_enum_schema or optional_enum_schema, and callers of TokioChildProcess::split should use its Transport implementation.

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

[tsarlandie-oai]

LGTM