Add support for DECIMAL(M, D) columns in MySQL EET - #1352

Summary

The EET oracle's type inference previously required FLOAT/DOUBLE/DECIMAL columns to be created without an (M, D) precision/scale specifier, because its type-pinning CAST (used for the redundant branch of transformation rules No. 3–4) could only target the bare types.

This PR removes the exclusion for DECIMAL by tracking a column's exact (M, D) end-to-end and reproducing it as CAST(... AS DECIMAL(M, D)). FLOAT/DOUBLE remain excluded under EET. This is because FLOAT(M, D)/DOUBLE(M, D) is not a valid CAST target, so there is no way to reproduce such a column's exact type in a type-pinning cast (also note FLOAT(M, D)/DOUBLE(M, D) columns are deprecated anyway, but they still remain supported for all other oracles).

Changes

Schema — track scale (MySQLSchema.java)

• Added a scale field (+ getScale()) to MySQLColumn, read from information_schema.columns.NUMERIC_SCALE, alongside the already-read NUMERIC_PRECISION.
• The MySQLColumn constructor and its single call site now thread scale through. information_schema always reports a concrete (M, D) for a DECIMAL column, defaulting to (10, 0) when the column was declared without one.

Type domain — enrich CastType (MySQLCastOperation.java)

• MySQLCastOperation.CastType was changed from a plain enum into a small final value class. Its keyword kinds (SIGNED, UNSIGNED, CHAR, FLOAT, DOUBLE, DECIMAL) remain public static final singletons (so existing e.g. == CastType.SIGNED comparisons are unaffected), while CastType.decimal(m, d) produces DECIMAL instances carrying (M, D).
• Added getPrecision/getScale, equals/hashCode (over kind + (M, D)), and a toString that renders DECIMAL(M, D).

Transformer (MySQLEETTransformer.java)

• inferColumnType maps a DECIMAL column to CastType.decimal(column.getPrecision(), column.getScale()).
• commonType compares with .equals() instead of ==, so two DECIMAL subexpressions with matching (M, D) (distinct instances) share a type. If (M, D) differs, commonType conservatively returns null, as MySQL's precise aggregation rule in this case is not known.

Table generation (MySQLTableGenerator.java)

• DECIMAL has now been reverted to the original behaviour of calling optionallyAddPrecisionAndScale (i.e. (M, D) is now allowed under EET).
• FLOAT/DOUBLE route through a new optionallyAddFloatingPointPrecisionAndScale, which omits (M, D) only while EET is active.