[mrattle]

Summary

Extend the opt-in correlate_with_opaques response validation from single-key reads (#79, #84) to single-key writes. A correlated write sends a generated O token and checks the echoed token before reading anything else from the reply. Gating is the same as for reads: the client-level option, opaque_correlation_request_only, and per-request correlate_with_opaques.

Intended behaviour:

• Covered: set/set_cas, add, replace/replace_cas, append, prepend, incr, decr, and the writes made by fetch (add) and cas/cas! (set/add).
• Writes always use a generated token. They take no meta_flags, so there is no caller opaque to prefer.
• A mismatched or missing token on an accepted reply code (HD/NS/EX/NF for ms, VA/NF/NS/EX for ma) returns the operation's existing failure value (false, or nil for incr/decr). The connection is closed after the request, without retrying or down-marking. A rejected VA leaves its counter line unread. Other protocol errors are unchanged.
• The outcome of a rejected write is unknown: the server may still have applied it, so false here does not mean "not stored".
• Unchanged: deletes (delete, delete_cas, delete_multi), writes inside quiet/multi blocks (memcached suppresses their success replies, so there is nothing to check), and multi-key writes (set_multi).
• In the OpenTelemetry middleware, write spans now get request_opaque and the correlation_* failure attributes, kept out of the tags and attributes shared with other middleware, matching read spans.

Testing

• Integration tests against a fake server cover the client/request gating matrix for writes, rejection of stale and missing tokens on each write path, and connection recycling without down-marking.
• Against real memcached: every write reply code (HD, NS, NF, EX, VA) echoes the token and keeps the connection open, and deletes, quiet blocks, and multi-key writes stay uncorrelated.
• Unit tests for the request formatter, the write reply parsers (rejected bodies are not consumed), and trace scoping on storage spans.
• Full suite passes locally on Ruby 4.0.7 / memcached 1.6.45, excluding test_rack_session.rb, which does not load on Ruby 4.0 with or without this change (rack 2.2 requires the removed cgi/cookie).