[CELEBORN-2411][CIP22] Preserve Spark IO cipher transformation

[SparksFyz]

What changes were proposed in this pull request?

Preserve Spark's effective IO cipher transformation when SparkCryptoHandler
builds its minimized SparkConf.

Add bidirectional interoperability tests between SparkCryptoHandler and
Spark's native CryptoStreamUtils using AES/CBC/PKCS5Padding.

Why are the changes needed?

CryptoStreamUtils.toCryptoConf() only extracts
spark.io.encryption.commons.config.*, while Spark reads
IO_CRYPTO_CIPHER_TRANSFORMATION separately from SparkConf.

As a result, SparkCryptoHandler previously dropped a non-default
transformation and silently fell back to Spark's default
AES/CTR/NoPadding.

Existing self-round-trip tests did not detect this because both encryption
and decryption used the same minimized configuration.

Does this PR resolve a correctness bug?

• Yes

Does this PR introduce any user-facing change?

• Yes

A non-default Spark IO cipher transformation is now preserved by Celeborn.
The default AES/CTR/NoPadding behavior remains unchanged.

How was this patch tested?

• Added Celeborn encryption -> Spark CryptoStreamUtils decryption coverage.
• Added Spark CryptoStreamUtils encryption -> Celeborn decryption coverage.
• Spark 3.5 SparkCryptoHandlerSuiteJ: 10 tests passed.
• Spark 3.0 / Scala 2.12.10 / JDK 11 compilation passed.
• Spark 4.0 / Scala 2.13.16 / JDK 17 compilation passed.
• Spark 3.5 Spotless check passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.55%. Comparing base (07dde50) to head (69422a6).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3790      +/-   ##
============================================
+ Coverage     58.41%   58.55%   +0.15%     
  Complexity      229      229              
============================================
  Files           398      399       +1     
  Lines         27993    28054      +61     
  Branches       2734     2738       +4     
============================================
+ Hits          16349    16424      +75     
+ Misses        10446    10430      -16     
- Partials       1198     1200       +2     

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[zaynt4606]

Thanks for the fix. One suggestion on the tests: both new interop tests use AES/CBC/PKCS5Padding. It would be good to add one more case that uses the default cipher transformation (no explicit IO_CRYPTO_CIPHER_TRANSFORMATION set) to cover the default path and guard against future regressions there.

[SparksFyz]

Thanks for the suggestion. Updated. @zaynt4606

[Copilot]

Pull request overview

Preserves Spark’s effective IO cipher transformation when Celeborn’s SparkCryptoHandler builds a minimized SparkConf, and adds cross-implementation interoperability tests to prevent silent fallback to Spark’s default cipher.

Changes:

• Propagate spark.io.encryption.crypto.cipher.transformation into the minimized SparkConf used by SparkCryptoHandler.
• Add bidirectional interoperability tests between Celeborn SparkCryptoHandler and Spark CryptoStreamUtils, including a non-default AES/CBC/PKCS5Padding transformation and the default transformation.

Reviewed changes

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

File	Description
client-spark/common/src/main/java/org/apache/spark/shuffle/celeborn/SparkCryptoHandler.java	Copies Spark’s effective IO cipher transformation into the minimized SparkConf used for crypto stream creation.
client-spark/common/src/test/java/org/apache/spark/shuffle/celeborn/SparkCryptoHandlerSuiteJ.java	Adds Spark↔Celeborn crypto interoperability tests for both default and explicitly configured cipher transformations.

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[zaynt4606]

LGTM

[zaynt4606]

Thanks @SparksFyz for the contribution!