feat(framework): add onError hook integration

[qantrepreneur]

Summary

• Add handleRequest function that wraps the render pipeline with error handling
• Catches RedirectResponse throwables and returns redirect responses
• Catches HttpErrorResponse throwables and returns their error responses
• Delegates unknown errors to user-defined onError(error, request) hook
• Falls back to generic 500 response when no onError is defined
• Export handleRequest from package index

Test plan

• Redirect throwables are caught and returned as redirect Responses (302 + location header)
• HttpError throwables are caught and returned with correct status/body
• Unknown errors call onError(error, request) with correct arguments
• onError can return a custom Response
• Falls back to generic 500 when onError is not defined
• Successful renders pass through unchanged
• Async onError handlers are supported

Closes #15

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