fix: don't wrap errors in streamable http auth client

[jamadeo]

Motivation and Context

When using the auth client, the errors are wrapped, and matches like this don't work as expected:

rust-sdk/crates/rmcp/src/transport/streamable_http_client.rs

Lines 376 to 378 in 4c34b64

	Err(StreamableHttpError::SeverDoesNotSupportSse) => {
	tracing::debug!("server doesn't support sse, skip common stream");
	}

How Has This Been Tested?

Use an auth client to connect to a server that does not support events from a GET connection. When the server responds 405, this match fails and terminates the connection instead of just dropping the listener.

Breaking Changes

No breaking changes

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