fix(hono): Return the existing client on repeated init in Bun and Deno (#24520)

On Bun and Deno, `init()` warned when Sentry was already initialized but
then initialized again, replacing the client. Anything buffered on the
first client was dropped and its integrations stayed installed against
it.

Both now keep the warning and return the existing client, as the Node
entry already does. Node's behavior is unchanged (it only logs in debug
mode, since it is initialized from an instrument file); Cloudflare
initializes per request through `withSentry` and is not affected.

Closes #24049

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>