feat: backwards-compatible create_message overloads for SEP-1577

[felixweinberger]

Summary

Introduce method overloading for create_message to preserve backwards compatibility while supporting the new tools feature from SEP-1577.

• When called without tools, returns CreateMessageResult with single content (backwards compatible)
• When called with tools, returns CreateMessageResultWithTools which allows array content

Motivation and Context

SEP-1577 changes CreateMessageResult.content from single to single | list. This would be a breaking change for existing SDK users. To avoid requiring a major version bump (v2.0), this PR uses @overload decorators to return different types based on whether tools are provided.

Existing code that doesn't use tools continues to work unchanged:

result = await session.create_message(messages=..., max_tokens=100)
# result.content is single content - no changes needed
if result.content.type == "text":
    return result.content.text

How Has This Been Tested?

• Type tests pass with pyright
• 469 tests pass
• Updated test for tool use to use CreateMessageResultWithTools
• Added test for basic CreateMessageResult

Breaking Changes

None for v1.x users. Existing code that doesn't use tools continues to work unchanged.

Note: v2.0 will align CreateMessageResult with the schema (array content).

Types of changes

• New feature (non-breaking change which adds functionality)

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

[felixweinberger]

Also added this to the list of things to clean up for v2: #1253

[pcarleton]

note: this confused me (why not leave it out?), but its a kwarg, and is needed to discriminate between the overloads

[bhosmer-ant]

thank you for this!