ci: pin EXPLAIN to FORMAT=TRADITIONAL for MySQL 9.5+

[sidorares]

Problem

test/integration/connection/test-execute-nocolumndef.test.mts fails against MySQL ≥ 9.5 (current mysql:9 / mysql:lts images), while passing on 5.7–9.0 and MariaDB.

Root cause

MySQL 9.5.0 changed the default of the explain_format system variable from TRADITIONAL to TREE (same change that hit rails/rails#55958). A bare EXPLAIN SELECT 1 now returns a single EXPLAIN column whose text embeds optimizer cost numbers:

-> Rows fetched before execution  (cost=0..0 rows=1)

The driver parses this resultset fine — only the test's hard-coded rows/fields expectations assume the traditional 12-column table. CI's mysql:9.0 job predates the default flip, which is why CI stayed green.

Fix

Pin the statement to explain format=traditional SELECT 1 on MySQL so the assertions stay deterministic. FORMAT=TRADITIONAL is accepted since MySQL 5.6 and emits no deprecation warning on 9.7.

MariaDB keeps the bare explain: it has no explain_format variable, its tabular default is unchanged, and its expectations were already branched via isMariaDB (I only verified FORMAT=TRADITIONAL support on MariaDB 12.3, not CI's 11.8, so the MariaDB path is deliberately untouched).

The test's actual purpose — the binary-protocol path from #130 / #37 where COM_STMT_PREPARE returns zero column definitions and fields only arrive at execute — is preserved: verified that prepare still returns 0 columns for the pinned statement on 8.3 and 9.7.

Verification

Test passes against all four local servers, npm run typecheck and npm run lint clean:

Server	Result
MySQL 5.7.44	✅
MySQL 8.3.0	✅
MySQL 9.7.2 (explain_format=TREE default)	✅
MariaDB 12.3.2	✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.45%. Comparing base (c86fe5a) to head (29310ca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4493   +/-   ##
=======================================
  Coverage   92.45%   92.45%           
=======================================
  Files          93       93           
  Lines       15855    15855           
  Branches     2254     2254           
=======================================
  Hits        14658    14658           
  Misses       1197     1197           

Flag	Coverage Δ
compression-0	92.06% <ø> (ø)
compression-1	92.43% <ø> (ø)
static-parser-0	91.26% <ø> (ø)
static-parser-1	91.37% <ø> (ø)
tls-0	92.01% <ø> (ø)
tls-1	92.45% <ø> (ø)

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