Fix query macros against CockroachDB by gating plan_cache_mode on EXPLAIN support

[BastienClement]

Problem

Since 0.9.0 the compile-time query macros (query!, query_as!,
query_scalar!) fail against CockroachDB:

error: error returned from database: unimplemented: SET usage inside a
function definition (SQLSTATE 0A000)

#3541 prepares the macros' describe connection by running
SET plan_cache_mode = 'force_generic_plan' inside a DO $$ … $$ block.
CockroachDB rejects SET inside a function/DO body at parse time, so the
block fails before its IF EXISTS guard runs. Closes #4274.

Fix

plan_cache_mode = force_generic_plan exists solely to keep EXPLAIN-based
nullability inference from being skewed by the NULL placeholder arguments
bound during describe. That inference already runs only when
is_explain_available() is true — false for CockroachDB, Materialize and
QuestDB (detected via their crdb_version / mz_version / questdb_version
parameter statuses).

The setting is moved out of the generic macro layer into PgConnection,
behind that same is_explain_available() check (plus a
server_version_num() >= 12 guard in place of the old pg_settings probe).
The macro layer now calls a per-database DatabaseExt::prepare_describe_connection
hook (a no-op for MySQL/SQLite). The setting is therefore skipped, by
construction, on every database that cannot use it — fixing CockroachDB and
pre-empting the same breakage on Materialize/QuestDB — while behaving exactly
as before on PostgreSQL.

Behaviour on standard PostgreSQL

Unchanged: the dedicated describe connection still gets force_generic_plan
(verified by the existing describe tests). describe() itself is untouched,
so no reused connection is affected.

Testing

• Existing tests/postgres/describe.rs suite passes, plus a new
  it_forces_generic_plan_for_describe regression test.
• Verified manually against CockroachDB v26.2.1: before this change a query!
  macro fails with SQLSTATE 0A000; afterwards it compiles and plan_cache_mode
  is left untouched. CockroachDB is not in the CI matrix, so that half cannot
  be covered by an automated test; the fix is structural (gated on the
  pre-existing, already-tested CockroachDB detection).
• cargo clippy -- -D warnings and rustfmt --check are clean.