sql: introduce canary full stats rollout and `stats_as_of` session var#156385

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ZhouXing19 wants to merge 6 commits intocockroachdb:masterfrom

ZhouXing19:canary-main

Commits on Oct 30, 2025

Commits on Oct 31, 2025

sql/opt: implement canary full statistics rollout with configurable window
ZhouXing19
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sql: implement stats_as_of session variable for time-based statistics selection
ZhouXing19
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sql/opt: add tests for using canary stats rollout in makeTableStatistics
ZhouXing19
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