fix(go-models): route hosted OCR providers through drivers (#15233)

## Summary
- route hosted MinerU.Net and PaddleOCR.Net provider names to their
existing Go drivers
- add regression coverage for loading the hosted OCR provider configs
through ProviderManager

## What changed
- Added canonical provider-name aliases for the hosted OCR provider
display names.
- Covered both bundled configs with a focused provider-manager test.

## Why
The hosted provider configs use display names with `.Net`, while model
factory dispatch lowercases the provider name. Without aliases, those
configs fall through to `DummyModel` instead of using the existing
MinerU and PaddleOCR drivers.

---------

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

conf/models/mineru.json

@@ -7,7 +7,7 @@
     "doc_parse": "v4/extract/task",
     "tasks": ""
   },
-  "class": "mineru",
+  "class": "mineru.net",
   "models": [
     {
       "name": "vlm",
@@ -22,4 +22,4 @@
       ]
     }
   ]
-}
+}

conf/models/paddleocr.json

@@ -6,7 +6,7 @@
   "url_suffix": {
     "ocr": "v2/ocr/jobs"
   },
-  "class": "paddleocr",
+  "class": "paddleocr.net",
   "models": [
     {
       "name": "PaddleOCR-VL-1.5",
@@ -30,4 +30,4 @@
       ]
     }
   ]
-}
+}