fix: add default values to Literal type fields in content types

[felixweinberger]

Summary

Add default values to single-value Literal type fields in content types so users don't have to pass them explicitly.

Motivation and Context

Content types like TextContent and ImageContent have single-value Literal type fields (e.g., type: Literal["text"]) that should default automatically since there's only one valid value.

Before:

TextContent(type="text", text="hello")
ImageContent(type="image", data="...", mimeType="image/png")

After:

TextContent(text="hello")
ImageContent(data="...", mimeType="image/png")

Fixes #1731

How Has This Been Tested?

Added regression test test_content_type_literal_defaults in tests/test_types.py covering representative types (TextContent, ImageContent, PromptReference).

Breaking Changes

None. Existing code that explicitly passes the type field will continue to work.

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

Affected types:

• TextContent
• ImageContent
• AudioContent
• ToolUseContent
• ToolResultContent
• EmbeddedResource
• ResourceLink
• ResourceTemplateReference
• PromptReference

This follows the same pattern as PR #1292 which added defaults for method: Literal fields on request/notification types.