[BUG] WebSocket Upgrade header detection is case-sensitive, violating RFC 6455

## Description
`String upgrade = headers.getFirst(UPGRADE); if (StringUtils.isNotEmpty(upgrade) && RpcTypeEnum.WEB_SOCKET.getName().equals(upgrade))`. `RpcTypeEnum.WEB_SOCKET.getName()` is the lowercase literal `"websocket"` and the comparison is case-sensitive. RFC 6455 4.2.1/4.1 requires the `Upgrade` header value to be compared case-insensitively. A client sending `Upgrade: WebSocket` or `Upgrade: WEBSOCKET` fails the check, so `rpcType` defaults to `HTTP`.

## Location
```
shenyu-plugin-global/.../DefaultShenyuContextBuilder.java:72-75
```

## Impact
Standards-compliant WebSocket clients that emit a non-lowercase `Upgrade` value fail to be proxied as WebSocket; `WebSocketPlugin.skip()` returns `true` and the upgrade handshake is not proxied. Most browsers send lowercase so impact is limited to non-browser/edge clients; still a real correctness deviation.

## Suggested fix
Use `equalsIgnoreCase`, e.g. `RpcTypeEnum.WEB_SOCKET.getName().equalsIgnoreCase(upgrade)`.

## Related existing issue(s)
None

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start at shenyu-plugin-global/.../DefaultShenyuContextBuilder.java:72-75, where the Upgrade header is compared, and trace how WebSocketPlugin.skip() uses the resulting rpcType. Make Upgrade value matching accept the RFC-required capitalization variants, then verify that WebSocket clients using WebSocket or WEBSOCKET are proxied as WebSocket rather than treated as HTTP.

Written by the indexing model from the issue text.