[Task] Extract shared upstream-data handler base across divide/websocket/grpc (copy-paste + magic strings)

## Description
Byte-identical `DiscoveryUpstreamData -> Upstream` conversion (Properties-parse -> `Upstream.builder()` with inline `"warmup"/"10"/"gray"/"false"/"healthCheckEnabled"/"true"` literals) duplicated across divide/websocket/grpc. The surrounding null-check->convert->split-gray->submit body is also duplicated verbatim between divide and websocket. The dubbo family already has `AbstractDubboPluginDataHandler` proving the abstraction; divide/websocket/grpc don't use one.

## Location
```
shenyu-plugin-divide/.../DivideUpstreamDataHandler.java:66
shenyu-plugin-websocket/.../WebSocketUpstreamDataHandler.java:59
shenyu-plugin-grpc/.../GrpcDiscoveryUpstreamDataHandler.java:65
(ApacheDubboProxyService.java:171 duplicates conversion a 2nd time within dubbo)
```

## Impact
A bug fix (e.g. warmup default) must be applied 3+ times; the inline magic strings risk per-plugin drift. A typo at one site silently breaks gray/warmup parsing for that plugin only.

## Suggested fix
Lift to a shared `AbstractDiscoveryUpstreamDataHandler` mirroring dubbo's pattern; extract an `UpstreamProps` constant/typed deserializer.

## 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)._

Read the handlers at the listed divide, websocket, and grpc paths, then compare them with AbstractDubboPluginDataHandler and the conversion in ApacheDubboProxyService.java. Trace the duplicated conversion and submission flow first; done means the shared handler and typed property deserialization cover the identified sites without repeated magic-string conversion logic.

Written by the indexing model from the issue text.