fix(purchase): follow-ups to the provider error summaries (value slots, failure email, Azure 422 text, docs) #1748

Summary

Follow-ups from the reviews of #1739 (provider error summaries on the exchange, marketplace, refund and collection paths). None of these blocked the merge.

• Quoted value slots. shortValuePattern in internal/purchase/errnotes.go accepts any short token in the quoted slots of allowlisted notes, such as a reserved ceiling or an unknown provider name. That includes dotted-quad IPs, 12-digit numbers and dotted host names. Today those values come from CUDly's own config and database, but limiting the slot to the value's real shape would keep a future writer from showing something else.
• Purchase-failed email. The failure email's Detail field carries the raw execution error. It should use the category message and the execution reference, like the API responses.
• Azure 422 refusal text. An Azure policy refusal returned as 422 passes the provider's policy text through. It should use a fixed message, with the provider text in the server log.
• Stale doc comments. A few comments still describe the old pass-through behaviour of the error helpers. Update them to match the closed-note model.

Related

#1688, #1702, #1721.