fix: tighten input_schema per Volcengine Jimeng 3.0 Pro contract

- frames: enum [121, 241] (was minimum 1)
- prompt: maxLength 800 (was 2000)
- seed: minimum -1 (was unbounded)
- Add 9 schema validation rejection tests
- Add authoritative API reference link to PROVIDERS.md
- Document CVSync2Async* route choice and schema constraints

Fixes calesthio's second review feedback on PR #341.

tools/video/jimeng_video.py

@@ -86,7 +86,8 @@ class JimengVideo(BaseTool):
         "properties": {
             "prompt": {
                 "type": "string",
-                "description": "Video description. Max 2000 chars. Supports Chinese.",
+                "maxLength": 800,
+                "description": "Video description. Max 800 chars. Supports Chinese.",
             },
             "operation": {
                 "type": "string",
@@ -102,7 +103,7 @@ class JimengVideo(BaseTool):
             },
             "frames": {
                 "type": "integer",
-                "minimum": 1,
+                "enum": [121, 241],
                 "default": 121,
                 "description": "Total frames. 121=5s, 241=10s at 24fps.",
             },
@@ -113,6 +114,7 @@ class JimengVideo(BaseTool):
             },
             "seed": {
                 "type": "integer",
+                "minimum": -1,
                 "default": -1,
                 "description": "Random seed. -1 for random.",
             },