chore: create union for working with message content

[LucaButBoring]

Moves message content types into a union and uses that throughout the codebase. Suggested by @Kludex in #725.

Motivation and Context

Simplifies code, makes for fewer places to update when new content types are introduced in the future.

How Has This Been Tested?

Unit tests

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

• Used a raw union instead of RootModel as suggested at the top of the file to avoid breaking changes. Using a new type extending RootModel would require wrapping all existing usages of TextContent etc. in that new type.
• Deliberately not used in sampling as sampling messages don't support embedded resources right now (see Enforce parity between supported message types in tool results and sampling modelcontextprotocol#522).

[Kludex]

I will accept this PR as I don't think anyone is importing this directly.