Fix TypeError on 403 OAuth error body with a string

[orthodoX]

Summary

A 403 from an OAuth-protected MCP server crashes the StreamableHTTP transport with
TypeError: String does not have #dig method instead of producing a usable error. This
turns token rejected signal into a crash that surfaces far from its
cause (the transport runs the request on a background thread, so the original backtrace is
lost and the re-raised error is a bare TransportError).

Root cause

handle_oauth_authorization_error assumes the 403 body is a JSON-RPC error and digs two
levels in:

error_message = error_body.dig("error", "message") || "Authorization failed"

But when a server's OAuth/resource layer rejects the token, the body is an RFC 6749 OAuth
error response (§5.2), where
error is a string, not a nested object:

{ "error": "invalid_token", "error_description": "The access token expired" }

Hash#dig("error", "message") then calls #dig on the String value of error and raises
TypeError: String does not have #dig method. The method's only rescue is
rescue JSON::ParserError, so the TypeError escapes uncaught.

This path is reached specifically for status_code == 403 && @oauth_provider when there is no
www-authenticate challenge header (handle_authorization_challenge →
handle_client_error → handle_oauth_authorization_error), which is exactly how many OAuth
resource servers respond to an expired/invalid token.

Reproduction

transport = RubyLLM::MCP::Native::Transports::StreamableHTTP.allocate
resp = Struct.new(:body).new('{"error":"invalid_token","error_description":"expired"}')
transport.send(:handle_oauth_authorization_error, resp, 403)
# Before: TypeError: String does not have #dig method
# After:  RubyLLM::MCP::Errors::TransportError:
#         Authorization failed (403 Forbidden). expired. Check token scope and permissions.

Backwards compatibility

None broken. The exception class and message format are preserved; the JSON-RPC branch
behaves exactly as before. The only change is that the previously-crashing OAuth-shaped body
now yields a normal TransportError.