refactor(BA-5978): add to_dict/from_error_details on SchemaValidationError

Per review feedback, expose the dict conversion as a method on the
dataclass rather than calling ``asdict`` at every site. Also expose
``from_error_details`` for the reverse direction so callers that catch
both ``BackendAISchemaValidationFailed`` and ``ValidationError`` can
normalize the latter into a ``SchemaValidationError`` list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jopemachine

src/ai/backend/common/exception.py

@@ -3,7 +3,7 @@
 import enum
 from abc import ABC, abstractmethod
 from collections.abc import Mapping
-from dataclasses import dataclass
+from dataclasses import asdict, dataclass
 from typing import Any, Self
 
 from aiohttp import web
@@ -458,6 +458,20 @@ class SchemaValidationError:
     msg: str
     url: str | None = None
 
+    @classmethod
+    def from_error_details(cls, details: ErrorDetails) -> Self:
+        """Build from a single ``pydantic_core.ErrorDetails`` entry."""
+        return cls(
+            type=details["type"],
+            loc=tuple(details["loc"]),
+            msg=details["msg"],
+            url=details.get("url"),
+        )
+
+    def to_dict(self) -> dict[str, Any]:
+        """JSON-friendly representation for ``extra_data`` / HTTP responses."""
+        return asdict(self)
+
 
 @dataclass(frozen=True)
 class SchemaValidationFailureInfo:

src/ai/backend/common/types.py

@@ -13,7 +13,7 @@
 from collections import UserDict, UserString, defaultdict, namedtuple
 from collections.abc import AsyncIterator, Iterable, Mapping, Sequence
 from contextvars import ContextVar
-from dataclasses import asdict, dataclass, field
+from dataclasses import dataclass, field
 from decimal import Decimal, InvalidOperation
 from functools import lru_cache
 from ipaddress import ip_address, ip_network
@@ -209,7 +209,7 @@ def build_validation_error(cls, info: SchemaValidationFailureInfo) -> BackendAIE
         :class:`BackendAISchemaValidationFailed`."""
         return BackendAISchemaValidationFailed(
             extra_msg=info.summary,
-            extra_data={"errors": [asdict(e) for e in info.errors]},
+            extra_data={"errors": [e.to_dict() for e in info.errors]},
         )
 
     @classmethod