Add unified RequestValidationError handler to unify error response schema

- Add RequestValidationError handler that converts Pydantic validation errors to unified ErrorResponse format
- Ensures all error responses return consistent schema: code, detail, metadata, correlation_id
- Add field_errors count and first_field location to metadata for validation errors
- Register handler in exception handler hierarchy before HTTPException handler
- Add comprehensive tests for validation error responses
- Update Backend-Development.md documentation to include correlation_id field and validation error details
- All 44 error-related tests pass (38 existing + 6 new validation tests)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/main.py

@@ -24,6 +24,7 @@ if TYPE_CHECKING:
 
 import structlog
 from fastapi import FastAPI, HTTPException, Request, status
+from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse, RedirectResponse
 from starlette.middleware.base import BaseHTTPMiddleware
@@ -621,6 +622,50 @@ async def _http_exception_handler(
     )
 
 
+async def _request_validation_error_handler(
+    request: Request,
+    exc: RequestValidationError,
+) -> JSONResponse:
+    """Return a standardized error response for Pydantic validation errors.
+
+    Converts FastAPI's RequestValidationError to our unified ErrorResponse format.
+    Aggregates validation errors into metadata for the client to handle.
+
+    Args:
+        request: The incoming FastAPI request.
+        exc: The :class:`fastapi.exceptions.RequestValidationError`.
+
+    Returns:
+        A :class:`fastapi.responses.JSONResponse` with status 400.
+    """
+    log.warning(
+        "request_validation_error",
+        path=request.url.path,
+        method=request.method,
+        error_count=len(exc.errors()),
+    )
+
+    validation_errors = exc.errors()
+    error_details: dict[str, str | int | float | bool | None] = {}
+
+    if validation_errors:
+        error_details["field_errors"] = len(validation_errors)
+        first_error = validation_errors[0]
+        error_details["first_field"] = ".".join(str(x) for x in first_error["loc"])
+
+    error_response = ErrorResponse(
+        code="invalid_input",
+        detail="Request validation failed.",
+        metadata=error_details,
+        correlation_id=_get_correlation_id(request),
+    )
+
+    return JSONResponse(
+        status_code=status.HTTP_400_BAD_REQUEST,
+        content=error_response.model_dump(),
+    )
+
+
 # ---------------------------------------------------------------------------
 # Setup-redirect middleware
 # ---------------------------------------------------------------------------
@@ -854,9 +899,10 @@ def create_app(settings: Settings | None = None) -> FastAPI:
     # 4. OperationError handler → HTTP 500
     # 5. ServiceUnavailableError handler → HTTP 503
     # 6. Generic DomainError handler (catch-all for any unregistered DomainError subclass) → HTTP 500
-    # 7. HTTPException (FastAPI built-ins, validation errors) → HTTP varies
-    # 8. ValueError (Pydantic validation) → HTTP 400
-    # 9. Exception (absolute catch-all for unexpected errors) → HTTP 500
+    # 7. RequestValidationError handler (Pydantic validation errors) → HTTP 400
+    # 8. HTTPException (FastAPI built-ins) → HTTP varies
+    # 9. ValueError (Pydantic validation) → HTTP 400
+    # 10. Exception (absolute catch-all for unexpected errors) → HTTP 500
     #
     # This ensures that any new DomainError subclass that inherits from a registered category
     # is automatically handled with the correct error_code and metadata. If a developer adds
@@ -872,6 +918,7 @@ def create_app(settings: Settings | None = None) -> FastAPI:
     app.add_exception_handler(OperationError, _domain_error_handler)  # type: ignore[arg-type]
     app.add_exception_handler(ServiceUnavailableError, _service_unavailable_handler)  # type: ignore[arg-type]
     app.add_exception_handler(DomainError, _domain_error_handler)  # type: ignore[arg-type]
+    app.add_exception_handler(RequestValidationError, _request_validation_error_handler)  # type: ignore[arg-type]
     app.add_exception_handler(HTTPException, _http_exception_handler)  # type: ignore[arg-type]
     app.add_exception_handler(ValueError, _value_error_handler)  # type: ignore[arg-type]
     app.add_exception_handler(Exception, _unhandled_exception_handler)