feat: Decimal128 write support

[killme2008]

Adds DataType.Decimal128 write support on the unary and streaming paths.

Close #11

What

• Table.addDecimalFieldColumn(name, precision, scale) declares a DECIMAL(precision, scale) FIELD column. The unary/streaming encoder ships a proto DecimalTypeExtension in the column schema, so server-side auto-create produces a DECIMAL(precision, scale) column.
• Values accept decimal strings (recommended — exact), numbers, or bigints, and are converted to the 128-bit unscaled integer GreptimeDB stores (round(value * 10^scale)). Excess fractional digits round half-up on the magnitude (away from zero for negatives, matching Java BigDecimal.HALF_UP).
• Range is validated against the declared precision after rounding (so e.g. 999.5 into DECIMAL(3,0) is rejected); malformed input, non-finite numbers, and invalid precision/scale throw ValueError.
• The 128-bit value is split into the proto Decimal128 { hi, lo } int64 pair using the standard two's-complement decomposition, matching the server's from_value_precision_scale reconstruction.
• Bulk Arrow path does not support decimal and throws ValueError (at both schema build and value encode), keeping unary/bulk rejection behavior explicit.

Tests

12 unit tests covering scaling/padding, half-up rounding (incl. negatives), number/bigint/exponential inputs, precision-overflow and malformed-input rejection, the hi/lo split (incl. negative two's-complement), and toProtoValue oneof + missing-metadata + null encoding.

typecheck / lint / format:check / build green; full suite 200 passing.

Notes

• Decimal is unary/streaming only by design; the bulk Arrow path is unchanged.
• Builds on top of main (post multi-endpoint-failover merge).

[Copilot]

Pull request overview

Adds SDK support for writing DECIMAL(precision, scale) values as DataType.Decimal128 on the unary and streaming row-insert paths, including column schema metadata via DecimalTypeExtension, plus unit tests and explicit bulk Arrow-path rejection.

Changes:

• Introduces DataType.Decimal128 and Table.addDecimalFieldColumn(name, precision, scale) for declaring decimal columns with precision/scale metadata.
• Encodes Decimal128 values into the protobuf decimal128Value oneof by converting inputs to an unscaled 128-bit integer and splitting into {hi, lo} parts.
• Makes the bulk Arrow path explicitly reject Decimal128 at both schema and value normalization stages; adds unit tests and updates the changelog.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/unit/decimal.test.ts	Adds unit coverage for decimal parsing/rounding, hi/lo splitting, and proto oneof encoding behavior.
src/write/encode.ts	Ships DecimalTypeExtension in column schema for Decimal128 and passes decimal metadata into value encoding.
src/table/value.ts	Adds Decimal128 handling in toProtoValue, converting to unscaled bigint and encoding {hi, lo}.
src/table/validators.ts	Implements decimal parsing/rounding (decimalToUnscaled) and {hi, lo} split (decimal128Parts).
src/table/table.ts	Adds addDecimalFieldColumn and enforces presence/consistency of decimal metadata for Decimal128 columns.
src/table/schema.ts	Extends ColumnSpec to carry decimal (precision/scale) metadata.
src/table/data-type.ts	Adds DataType.Decimal128 and maps it to proto ColumnDataType.DECIMAL128.
src/bulk/arrow-encoder.ts	Throws ValueError when Decimal128 is used on the bulk Arrow path.
package.json	Adds prepare script to build on install/publish workflows.
CHANGELOG.md	Documents the new Decimal128 write support and bulk-path behavior.

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[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.