sql: report RevalidateCachedQuery as the cached-plan error's routine

[demostheneslld]

During the Bind phase of the extended pgwire protocol (and Execute for the simple protocol), the "cached plan must not change result type" error is detected by clients via its Routine field (PG_DIAG_SOURCE_FUNCTION). PostgreSQL reports RevalidateCachedQuery, its plancache.c function. CockroachDB reported whichever internal function constructed the error - runExecBuilder until #164406 moved the extended-protocol check to Bind, which changed the value to execBind and silently broke clients matching the old name (e.g. activerecord-cockroachdb-adapter's is_cached_plan_failure?, which stopped translating the error into ActiveRecord's evict-and-retry recovery; we hit this in production).

This PR constructs the error in a dedicated function named RevalidateCachedQuery, so the reported routine is stable across refactors and identical to PostgreSQL's - detection heuristics written against PostgreSQL (ActiveRecord upstream matches exactly this name) work unchanged. The pgtest datadriven test now asserts Code, Message, and Routine (new keepErrRoutine option), making the fields clients key on a tested contract rather than an accident of the call stack. The assertion also holds when the suite runs against real PostgreSQL.

Compatibility note: this is itself a wire-behavior change, and it breaks two client classes once. (1) Clients that adapted to 26.2 by matching execBind stop matching when the pin ships. (2) Simple-protocol clients (SQL-level PREPARE/EXECUTE, no wire Bind) matching runExecBuilder work on every released version and break only here. Every alternative to freezing the accidental value breaks one of these groups eventually; the pin breaks them onto the value PostgreSQL has reported since 8.3, making it the last such break. Flagged in the release note; deferring to you on which release it rides.

Companion adapter fix: cockroachdb/activerecord-cockroachdb-adapter#403 switches detection to the error message, which works on pre- and post-#164406 servers. The two are complementary: the adapter fix covers existing versions, this makes the field reliable going forward.

Resolves the detection half of the issue behind #164406's driver-compat work.

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