hosting: http_exception_handler drops HTTPException.headers, so a route cannot set Vary, Cache-Control or WWW-Authenticate on an error response #369

Summary

The host's HTTPException handler rebuilds the response from status_code and detail only. Any headers a route attached to the exception are discarded. Starlette and FastAPI both honour HTTPException(headers=...) by default; the host's override silently loses them.

Observed

simple_module_hosting/_error_handlers.py:48-52 (0.0.26):

async def http_exception_handler(request: Request, exc: HTTPException) -> Response:
    if exc.status_code in _INERTIA_ERROR_STATUSES:
        detail = str(exc.detail) if exc.detail else ""
        return await render_error_page(request, exc.status_code, detail)
    return JSONResponse(status_code=exc.status_code, content={"detail": exc.detail})

Reproduction with any route:

@router.get("/x")
async def x():
    raise HTTPException(404, "nope", headers={"Cache-Control": "no-store", "Vary": "X-Tenant-ID"})

curl -i http://localhost:8000/api/<module>/x
HTTP/1.1 404 Not Found
content-type: application/json
...            # neither Cache-Control nor Vary is present

Why it matters

A public, cacheable read API on a multi-tenant host has to mark its 404s Cache-Control: no-store and Vary: <tenant header>, otherwise a shared cache keyed on the URL serves one tenant's 404 for another tenant's existing record. The same mechanism is how 401 carries WWW-Authenticate and 429 carries Retry-After. None of those can be set through the exception today.

Workaround in the records module

antosubash/smpy_modules#37 installs a custom APIRoute class (PublicErrorRoute) on its public router that catches HTTPException before the host handler and renders the JSON body itself so the headers survive. Every module wanting a header on an error response has to repeat that.

Proposed fix

    return JSONResponse(status_code=exc.status_code, content={"detail": exc.detail}, headers=exc.headers)

and pass exc.headers through render_error_page for the Inertia branch as well.