fix: detect OAuth requirement from unexpected content-type responses

Some MCP servers (e.g., Atlassian) return text/plain error responses
instead of HTTP 401 when unauthenticated. Add "unexpected content type"
to OAuth detection heuristics so these are correctly routed to the
OAuth flow rather than surfaced as generic connection errors.

Signed-off-by: Mohammod Al Amin Ashik <maa.ashik00@gmail.com>

Author: its-mash

crates/mcpmux-gateway/src/pool/transport/http.rs

@@ -94,6 +94,7 @@ impl HttpTransport {
             "access token",
             "missing or invalid",
             "bearer",
+            "unexpected content type",
         ];
         oauth_indicators.iter().any(|s| error_lower.contains(s))
     }
@@ -845,6 +846,13 @@ mod tests {
         assert!(HttpTransport::requires_oauth("transport channel closed"));
     }
 
+    #[test]
+    fn test_requires_oauth_unexpected_content_type() {
+        assert!(HttpTransport::requires_oauth(
+            "Unexpected content type: Some(\"text/plain;charset=UTF-8\")"
+        ));
+    }
+
     #[test]
     fn test_requires_oauth_false_for_unrelated() {
         assert!(!HttpTransport::requires_oauth("connection refused"));