fix: duplicate meta serialization

[DaleSeo]

Fixes #647

Motivation and Context

PR #617 (SEP-1319) added _meta fields to params structs, but WithMeta already injects _meta from extensions via #[serde(flatten)]. When both are populated, the serialized JSON contains duplicate _meta keys, which is invalid.

This PR replaces the derived Serialize on WithMeta with a manual implementation that extracts _meta from the flattened params, merges it with extensions _meta, and emits a single key. The fix is centralized in one place and covers all affected types. On conflict, extensions-level values take priority. No API changes; deserialization is unchanged.

How Has This Been Tested?

Add unit tests to verify the new behavior and prevent any regression.

Breaking Changes

None.

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