[AURON #1712] Fix Casting from Map to Utf8 not supported

[cxzl25]

Which issue does this PR close?

Closes #1712

Rationale for this change

Arrow error: Cast error: Casting from Map to Utf8 not supported

What changes are included in this PR?

Are there any user-facing changes?

How was this patch tested?

Add UT

[Copilot]

Pull request overview

This PR implements support for casting Map types to Utf8 (String) in the Auron query engine, addressing issue #1712. The implementation follows Spark-compatible formatting, where maps are rendered as {key1 -> value1, key2 -> value2}, and properly handles nested types and null values.

Key Changes:

• Added map-to-string casting logic in the Rust cast implementation that recursively converts keys and values to strings
• Added comprehensive unit tests in both Rust (4 tests) and Scala (4 tests) covering basic maps, null maps, empty maps, and nested/complex maps
• The implementation mirrors the existing struct-to-string casting pattern for consistency

Reviewed changes

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

File	Description
native-engine/datafusion-ext-commons/src/arrow/cast.rs	Adds map-to-string casting implementation (lines 222-267) with Spark-compatible formatting and 4 comprehensive unit tests covering various map scenarios
spark-extension-shims-spark/src/test/scala/org.apache.auron/AuronQuerySuite.scala	Adds 4 integration tests for map-to-string casting, properly gated for Spark 3.1+, covering simple maps, nested maps, maps with struct values, and empty maps

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