feat: rework Exception/ApiError (#107)

* feat: rework Exception/ApiError

* docs: update exceptions module

* fix: docstring

docs/module/exceptions.md

@@ -21,30 +21,37 @@ init_exceptions_handlers(app=app)
 This registers handlers for:
 
 - [`ApiException`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ApiException) — all custom exceptions below
+- `HTTPException` — Starlette/FastAPI HTTP errors
 - `RequestValidationError` — Pydantic request validation (422)
 - `ResponseValidationError` — Pydantic response validation (422)
 - `Exception` — unhandled errors (500)
 
+It also patches `app.openapi()` to replace the default Pydantic 422 schema with a structured example matching the `ErrorResponse` format.
+
 ## Built-in exceptions
 
 | Exception | Status | Default message |
 |-----------|--------|-----------------|
 | [`UnauthorizedError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.UnauthorizedError) | 401 | Unauthorized |
 | [`ForbiddenError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ForbiddenError) | 403 | Forbidden |
-| [`NotFoundError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NotFoundError) | 404 | Not found |
+| [`NotFoundError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NotFoundError) | 404 | Not Found |
 | [`ConflictError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ConflictError) | 409 | Conflict |
-| [`NoSearchableFieldsError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NoSearchableFieldsError) | 400 | No searchable fields |
-| [`InvalidFacetFilterError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.InvalidFacetFilterError) | 400 | Invalid facet filter |
+| [`NoSearchableFieldsError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NoSearchableFieldsError) | 400 | No Searchable Fields |
+| [`InvalidFacetFilterError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.InvalidFacetFilterError) | 400 | Invalid Facet Filter |
+| [`InvalidOrderFieldError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.InvalidOrderFieldError) | 422 | Invalid Order Field |
+
+### Per-instance overrides
+
+All built-in exceptions accept optional keyword arguments to customise the response for a specific raise site without changing the class defaults:
+
+| Argument | Effect |
+|----------|--------|
+| `detail` | Overrides both `str(exc)` (log output) and the `message` field in the response body |
+| `desc` | Overrides the `description` field |
+| `data` | Overrides the `data` field |
 
 ```python
-from fastapi_toolsets.exceptions import NotFoundError
-
-@router.get("/users/{id}")
-async def get_user(id: int, session: AsyncSession = Depends(get_db)):
-    user = await UserCrud.first(session=session, filters=[User.id == id])
-    if not user:
-        raise NotFoundError
-    return user
+raise NotFoundError(detail="User 42 not found", desc="No user with that ID exists in the database.")
 ```
 
 ## Custom exceptions
@@ -58,12 +65,51 @@ from fastapi_toolsets.schemas import ApiError
 class PaymentRequiredError(ApiException):
     api_error = ApiError(
         code=402,
-        msg="Payment required",
+        msg="Payment Required",
         desc="Your subscription has expired.",
-        err_code="PAYMENT_REQUIRED",
+        err_code="BILLING-402",
     )
 ```
 
+!!! warning
+    Subclasses that do not define `api_error` raise a `TypeError` at **class creation time**, not at raise time.
+
+### Custom `__init__`
+
+Override `__init__` to compute `detail`, `desc`, or `data` dynamically, then delegate to `super().__init__()`:
+
+```python
+class OrderValidationError(ApiException):
+    api_error = ApiError(
+        code=422,
+        msg="Order Validation Failed",
+        desc="One or more order fields are invalid.",
+        err_code="ORDER-422",
+    )
+
+    def __init__(self, *field_errors: str) -> None:
+        super().__init__(
+            f"{len(field_errors)} validation error(s)",
+            desc=", ".join(field_errors),
+            data={"errors": [{"message": e} for e in field_errors]},
+        )
+```
+
+### Intermediate base classes
+
+Use `abstract=True` when creating a shared base that is not meant to be raised directly:
+
+```python
+class BillingError(ApiException, abstract=True):
+    """Base for all billing-related errors."""
+
+class PaymentRequiredError(BillingError):
+    api_error = ApiError(code=402, msg="Payment Required", desc="...", err_code="BILLING-402")
+
+class SubscriptionExpiredError(BillingError):
+    api_error = ApiError(code=402, msg="Subscription Expired", desc="...", err_code="BILLING-402-EXP")
+```
+
 ## OpenAPI response documentation
 
 Use [`generate_error_responses`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.generate_error_responses) to add error schemas to your endpoint's OpenAPI spec:
@@ -78,8 +124,7 @@ from fastapi_toolsets.exceptions import generate_error_responses, NotFoundError,
 async def get_user(...): ...
 ```
 
-!!! info
-    The pydantic validation error is automatically added by FastAPI.
+Multiple exceptions sharing the same HTTP status code are grouped under one entry, each appearing as a named example keyed by its `err_code`. This keeps the OpenAPI UI readable when several error variants map to the same status.
 
 ---