error-handling-rfc9457

# RFC 9457 Problem Details

Standardize API error responses with machine-readable problem details.

## RFC 9457 vs RFC 7807

| Feature | RFC 7807 (Old) | RFC 9457 (Current) |
|---------|----------------|---------------------|
| Status | Obsolete | Active Standard |
| Multiple problems | Not specified | Explicitly supported |
| Error registry | No | Yes (IANA registry) |
| Extension fields | Implicit | Explicitly allowed |

## Problem Details Schema

```python
from pydantic import BaseModel, Field, HttpUrl
from typing import Any

class ProblemDetail(BaseModel):
    """RFC 9457 Problem Details for HTTP APIs."""

    type: HttpUrl = Field(
        default="about:blank",
        description="URI identifying the problem type"
    )
    title: str = Field(
        description="Short, human-readable summary"
    )
    status: int = Field(
        ge=400, le=599,
        description="HTTP status code"
    )
    detail: str | None = Field(
        default=None,
        description="Human-readable explanation specific to this occurrence"
    )
    instance: str | None = Field(
        default=None,
        description="URI reference identifying the specific occurrence"
    )

    model_config = {"extra": "allow"}  # Allow extension fields
```

## FastAPI Integration

### Exception Classes

```python
from fastapi import HTTPException
from typing import Any

class ProblemException(HTTPException):
    """Base exception for RFC 9457 problem details."""

    def __init__(
        self,
        status_code: int,
        problem_type: str,
        title: str,
        detail: str | None = None,
        instance: str | None = None,
        **extensions: Any,
    ):
        self.problem_type = problem_type
        self.title = title
        self.detail = detail
        self.instance = instance
        self.extensions = extensions
        super().__init__(status_code=status_code, detail=detail)

    def to_problem_detail(self) -> dict[str, Any]:
        result = {
            "type": se