feat: deprecated datetime

[killme2008]

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[Copilot]

Pull request overview

This PR deprecates ColumnDataType.DateTime (treated by GreptimeDB as an alias of TIMESTAMP_MICROSECOND) and removes internal special-casing by normalizing the type at the TableBuilder boundary.

Changes:

• Mark ColumnDataType.DateTime as [Obsolete] with updated XML docs, and keep timestamp validation accepting it.
• Normalize ColumnDataType.DateTime to ColumnDataType.TimestampMicrosecond inside TableBuilder.AddColumn.
• Remove DateTime handling from internal coercion/Arrow/proto mapping and update tests accordingly.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/GreptimeDB.Ingester.Tests/TypeCoercionTests.cs	Updates a coercion test to target TimestampMicrosecond instead of DateTime.
tests/GreptimeDB.Ingester.Tests/RequestBuilderTests.cs	Updates request-building test schema to use TimestampMicrosecond.
tests/GreptimeDB.Ingester.Tests/RecordBatchBuilderTests.cs	Updates record-batch test schema to use TimestampMicrosecond.
tests/GreptimeDB.Ingester.Tests/ArrowTypeMapperTests.cs	Removes DateTime coverage from Arrow type mapping tests.
src/GreptimeDB.Ingester/Types/TypeCoercion.cs	Removes DateTime alias coercion path; timestamps now handled only via explicit timestamp types.
src/GreptimeDB.Ingester/Types/ColumnDataType.cs	Adds deprecation docs + [Obsolete] for DateTime; keeps IsTimestamp() accepting it.
src/GreptimeDB.Ingester/Table/TableBuilder.cs	Normalizes DateTime to TimestampMicrosecond during column creation.
src/GreptimeDB.Ingester/Internal/RequestBuilder.cs	Removes DateTime proto encoding/mapping since it should be normalized upstream.
src/GreptimeDB.Ingester/Arrow/RecordBatchBuilder.cs	Removes DateTime arrow array building case.
src/GreptimeDB.Ingester/Arrow/ArrowTypeMapper.cs	Removes DateTime arrow type mapping and timestamp-unit handling.

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