[GH-2830] Adds Geography dual-dispatch to ST_Length

[zhangfengcdt]

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 #<issue_number>

What changes were proposed in this PR?

• Adds Geography dual-dispatch to ST_Length — returns geodesic length in meters on the WGS84 spheroid for Geography inputs.
• Use S2 directly to calculate the length directly.

How was this patch tested?

• mvn clean test -pl common -am -Dtest=FunctionTest
• New Spark SQL cases in GeographyFunctionTest
• mvn spotless:apply run before commit; no format violations.

Did this PR include necessary documentation updates?

• Yes, I have updated the documentation.

[Copilot]

Pull request overview

Adds Geography support for ST_Length so Spark SQL can return geodesic length (meters, WGS84 spheroid) when the input is a Geography, while preserving existing planar Geometry behavior.

Changes:

• Add dual-dispatch for ST_Length in Spark SQL expression resolution (Geometry vs Geography).
• Implement org.apache.sedona.common.geography.Functions.length(Geography) using Spheroid.length(toJTS(g)).
• Add unit/integration tests and new Geography SQL docs page for ST_Length.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
spark/common/src/main/scala/org/apache/spark/sql/sedona_sql/expressions/Functions.scala	Adds Geography overload to ST_Length expression via inferred overload resolution.
common/src/main/java/org/apache/sedona/common/geography/Functions.java	Introduces geodesic length(Geography) implementation (WGS84 spheroid via Spheroid).
common/src/test/java/org/apache/sedona/common/Geography/FunctionTest.java	Adds core-module tests for geodesic length (equator/meridian/point/null).
spark/common/src/test/scala/org/apache/sedona/sql/geography/GeographyFunctionTest.scala	Adds Spark SQL integration tests for ST_Length on Geography.
docs/api/sql/geography/Geography-Functions/ST_Length.md	Adds Geography function documentation for ST_Length.
docs/api/sql/geography/Geography-Functions.md	Adds ST_Length entry to the Geography functions index.

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