fix(http): default 500 response leaks err.message across all three backends

[pathosDev]

The default error mapping in all three backends serializes the thrown error's message into the 500 response body:

• FastifyBackend.writeError
• ExpressBackend error middleware
• HonoBackend app.onError

For non-HttpError throws this leaks internal details (stack-adjacent messages, driver errors, file paths) to clients — an information-disclosure wart.

Fix

• Default 500 body should be a generic { error: 'Internal Server Error' }; the real message goes to the logger only.
• Operators who want a custom body already have ServerBuilder.withErrorHandler (feat(http): error-handling & fallback routes + HTML/XSS response utilities #352).

Should land before 1.0. Add a Security CHANGELOG entry.