[GH-3058] Add GeographyTypeSerializer to SedonaFlink

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• Yes, and the PR name follows the format [GH-XXX] my subject. Closes SedonaFlink: add GeographyTypeSerializer for the Geography type #3058.

What changes were proposed in this PR?

This is the first step toward Geography type support in SedonaFlink (#3054).

It adds GeographyTypeSerializer, a Flink TypeSerializer<Geography> that lets Geography values flow through the Flink Table/DataStream type system, mirroring the existing GeometryTypeSerializer:

• Length-prefixed, null-safe binary layout, delegating to org.apache.sedona.common.S2Geography.GeographyWKBSerializer (the same serializer the Spark GeographyUDT uses), so geography bytes are interchangeable across engines.
• copy(Geography) is implemented via a serialize/deserialize round-trip, since Geography exposes no copy()/clone().
• A TypeSerializerSnapshot is provided for state schema compatibility, mirroring GeometrySerializerSnapshot.

No user-facing functions are registered yet — the ST_Geog* constructors and Catalog registration follow in a subsequent PR.

How was this patch tested?

Added GeographyTypeSerializerTest, which round-trips geographies through the Flink DataOutputView/DataInputView API:

• point and polygon round-trips (asserting EWKT and SRID are preserved),
• null round-trip,
• copy() for a value and for null.

mvn -pl flink test -Dtest=GeographyTypeSerializerTest → 4 tests pass.

Did this PR include necessary documentation updates?

• No, this PR does not affect any public API so no need to change the documentation. The user-facing geography functions and their docs are tracked as follow-up sub-tasks of Add Geography type support to SedonaFlink (parity with Spark) #3054.

[Copilot]

Pull request overview

This PR adds initial Flink runtime support for Sedona’s Geography type by introducing a dedicated Flink TypeSerializer<Geography> and accompanying unit tests, mirroring the existing geometry serializer approach in the Flink module.

Changes:

• Added GeographyTypeSerializer that serializes Geography values as a null-safe, length-prefixed byte payload using GeographyWKBSerializer.
• Added a TypeSerializerSnapshot implementation for Flink state/schema compatibility.
• Added GeographyTypeSerializerTest covering point/polygon round-trips, null handling, and copy() behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
flink/src/main/java/org/apache/sedona/flink/GeographyTypeSerializer.java	Introduces a Flink TypeSerializer + snapshot for Sedona Geography, using WKB-based encoding with a length prefix and null sentinel.
flink/src/test/java/org/apache/sedona/flink/GeographyTypeSerializerTest.java	Adds unit tests validating serialization/deserialization round-trips and copy() behavior for Geography.

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