feat: support EXPLAIN ANALYZE on SQL Server for actual execution plans

[DANze11]

Problem

On SQL Server, EXPLAIN maps to SET SHOWPLAN_XML, which compiles a statement
without executing it. The resulting plan carries estimates only — there is no way
to get a plan with real row counts, which is what you need when the estimates are
the problem.

There is also an existing gap. The read-only classifier already recognises
EXPLAIN ANALYZE (it is a PostgreSQL form, see utils/allowed-keywords.ts) and
correctly refuses it for DML, but the SQL Server connector has no handling for it.
Both spellings currently break:

EXPLAIN ANALYZE SELECT ...    -- reaches the server as `ANALYZE SELECT ...`
EXPLAIN (ANALYZE) SELECT ...  -- reaches the server as `(ANALYZE) SELECT ...`

Both are syntax errors.

Solution

EXPLAIN ANALYZE maps to SET STATISTICS XML, which executes the statement and
returns a plan carrying ActualRows / ActualExecutions — the PostgreSQL
EXPLAIN ANALYZE contract this connector already emulates for plain EXPLAIN.

EXPLAIN SELECT ...          -- estimated plan (unchanged)
EXPLAIN ANALYZE SELECT ...  -- actual plan, real row counts

Notes on the implementation

Plan extraction reads the last recordset, not recordset (singular).
STATISTICS XML interleaves each statement's plan with that statement's own
result sets, so the plan sits at no fixed index — the singular accessor would hand
back the query's data instead of the plan.

Option parsing covers the forms the classifier already recognises: ANALYZE,
ANALYZE VERBOSE, (ANALYZE), (ANALYZE, ...), (ANALYZE false).
PostgreSQL-only options (BUFFERS, VERBOSE, COSTS, …) have no SQL Server
counterpart and are refused by name rather than silently dropped — quietly
ignoring an option would return something other than what was asked for. A
disabled ANALYZE (false/off/0) falls back to the estimated plan, mirroring
the classifier's own negative lookahead.

Read-only is handled explicitly. Unlike EXPLAIN, EXPLAIN ANALYZE executes,
so it is not inherently read-only the way explainQuery() is. Under
options.readonly the statement runs inside a transaction that always rolls back,
reusing the reasoning and keyword guards of executeReadOnly() (#342, #350).

Session isolation is preserved. The SET STATISTICS XML toggle runs on the
same short-lived single-connection pool explainQuery() already uses, so the
session state cannot leak onto a shared pool connection where a concurrent query
would inherit it.

Tests

12 new integration tests against a real SQL Server container, covering: actual
plan contents (RunTimeInformation / ActualRows), that the statement really
executes (unlike EXPLAIN), rollback under readonly, the parenthesized forms,
disabled ANALYZE falling back to an estimated plan, refusal of PostgreSQL-only
options, and the malformed cases.

Tests  62 passed (62)

The full sqlserver.integration.test.ts suite passes (50 pre-existing + 12 new).
No changes to other connectors; pnpm test:unit is unchanged.

[Copilot]

Pull request overview

Adds SQL Server support for PostgreSQL-style EXPLAIN ANALYZE by translating it to SET STATISTICS XML, enabling actual execution plans (with real row counts) while preserving read-only safety via rollback-on-complete execution when options.readonly is enabled.

Changes:

• Extend SQL Server executeSQL() EXPLAIN handling to parse EXPLAIN modifiers and route EXPLAIN ANALYZE to a new SET STATISTICS XML execution path.
• Implement plan extraction from STATISTICS XML results by scanning the final recordsets for a ShowPlanXML payload.
• Add SQL Server integration tests covering EXPLAIN ANALYZE semantics, readonly rollback behavior, supported/unsupported option forms, and malformed inputs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/connectors/sqlserver/index.ts	Adds EXPLAIN option parsing and EXPLAIN ANALYZE execution-plan support via SET STATISTICS XML, including readonly rollback handling and plan extraction.
src/connectors/tests/sqlserver.integration.test.ts	Adds SQL Server integration coverage for actual-plan generation, execution vs. compile-only behavior, and option/error handling.