add type field to client session config BaseModels

[bfineran]

Adds a pre-set type field to SseServerParameters, StreamableHttpParameters, and StdioServerParameters so serialized parameters will easily be loaded into the correct class.

Motivation and Context

All fields of SseServerParameters overlap with StreamableHttpParameters, so a third party application that would want to connect to multiple session with a config such as:

class ClientConfig(BaseModel):
    mcp_configs: List[Union[StreamableHttpParameters, SseServerParameters]]

would not have a way to be instantiated from a serialized config (like a yaml file) since the following is ambiguous.

mcp_configs:
  - url: ""

Additionally, if someone were to serialize either the sse or http config classes they would not be able to reload the second class listed in the union.

Adding the preset type field should fix this in a backwards compatible way.

How Has This Been Tested?

Existing tests cover backwards compatibility, confirmed this PR addresses the described use case

Breaking Changes

No, added fields include the only valid Literal default.

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