fix(mssql): map bare Stddev to STDEV, not STDEVP

[00200200]

Summary

Closes #12057.

The MSSQL dialect mapped bare sge.Stddev to STDEVP (population). sqlglot parses T-SQL STDEV — the sample function — into that node, so a same-dialect re-render silently changed the statistic.

sge.StddevPop already renders STDEVP, and three lines below sge.Variance already maps to the sample VAR, so this looks like a typo rather than an intentional choice. Vanilla sqlglot's tsql dialect already round-trips STDEV correctly.

Reproduction path

Backend.sql(query) without dialect= returns early and does not transpile. The path that does hit this is Table.sql() / add_query_to_expr, which re-renders unconditionally:

# before: SELECT STDEVP([x]) AS [s] FROM [t]
# after:  SELECT STDEV([x]) AS [s] FROM [t]
sqlglot.transpile("SELECT STDEV([x]) AS [s] FROM [t]", read=MSSQL, write=MSSQL)

Ibis's own t.x.std(how="sample"|"pop") compilation already went through StddevSamp / StddevPop and was unaffected; the tests lock that in.

Test plan

• pytest ibis/backends/sql/tests/test_compiler.py::test_mssql_stdev_roundtrips_as_sample -o addopts=
• pytest ibis/backends/sql/tests/test_compiler.py::test_mssql_std_how_emits_distinct_functions -o addopts=
• Confirmed the round-trip assertion fails if the mapping is reverted to stdevp

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