[Feature Request]: ClickHouseIO: Add Decimal(P, S) support for fixed-point columns #39840

What would you like to happen?

Problem

ClickHouseIO has no notion of ClickHouse's fixed-point Decimal family — Decimal(P, S),
Decimal(P), bare Decimal, and the width aliases Decimal32(S), Decimal64(S),
Decimal128(S), Decimal256(S). The column-type parser rejects these type strings, so
ClickHouseIO.getTableSchema() throws on the DESCRIBE TABLE output and pipelines cannot
write to any table containing a Decimal column — a blocker for monetary and other
fixed-point data.

Proposed solution

1. Schema model

   • Add TypeName.DECIMAL.
   • Extend ColumnType with a scale field alongside the existing precision field
     (introduced for DateTime64), plus a validated factory
     ColumnType.decimal(int precision, int scale) (P in [1, 76], S in [0, P]).

2. Parser

   • Grammar rules for Decimal[(P[, S])] (bare Decimal = Decimal(10, 0),
     Decimal(P) = Decimal(P, 0), matching ClickHouse) and the width aliases
     Decimal32/64/128/256(S) (precisions 9/18/38/76), reachable through
     Nullable(...) and Array(...).

3. Beam field type mapping

   • Decimal(P, S) maps to the FixedPrecisionNumeric.of(P, S) logical type
     (base type DECIMAL, values are BigDecimal), preserving the declared
     precision/scale through getEquivalentSchema — the same logical type JdbcIO
     uses for NUMERIC columns.

4. Writer

   • Serialize as ClickHouse's RowBinary representation: the unscaled value as a
     little-endian signed integer whose width (32/64/128/256 bits) is chosen from the
     declared precision, exactly per ClickHouse's Decimal(P, S) → Decimal32/64/128/256
     synonym rule.

5. Defaults

   • Support DEFAULT <literal> on Decimal columns in
     ColumnType.parseDefaultExpression — without it, schema fetch fails for any table
     with a defaulted Decimal column.

6. Tests

   • Parser/mapping unit tests, byte-level writer tests, and round-trip tests against the
     ClickHouse test container for all four width buckets, Nullable(Decimal),
     Array(Decimal), DEFAULT handling, and getTableSchema on the server's canonicalized
     DESCRIBE output.

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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