[LIVY-1083] Coerce batch/session Int fields sent as JSON strings

[soumyadeeplogin]

What changes were proposed in this pull request?

CreateBatchRequest and CreateInteractiveRequest declare driverCores, executorCores and numExecutors as Option[Int]. Because of Scala type erasure, Option[Int] erases to Option[Object], so jackson-module-scala cannot tell the element type is Integer. When a client sends one of these fields as a JSON string (e.g. "4" instead of 4), Jackson stores it verbatim as Some("4") without error. The bad value only surfaces later as a ClassCastException when the code unboxes it to Int while building the Spark submit command, which the servlet returns to callers as an opaque HTTP 500.

This PR annotates the three fields on both request classes with @JsonDeserialize(contentAs = classOf[java.lang.Integer]) so Jackson coerces numeric strings to Int during deserialization, and rejects truly non-numeric strings with a client-facing 400 (JsonMappingException) instead of a 500 later during app creation.

Why are the changes needed?

To fail fast with a clear 400 at the API boundary when a client sends a malformed numeric field, instead of an opaque 500 deep in Spark app submission logic.

Does this PR introduce any user-facing change?

Yes, but only for previously-broken input. POST /batches and interactive session creation now accept JSON string values for driverCores/executorCores/numExecutors (e.g. "4") by coercing them to the equivalent int, and return a 400 instead of a 500 for genuinely non-numeric strings (e.g. "notanumber", "4.5").

How was this patch tested?

Added unit tests to CreateBatchRequestSpec and CreateInteractiveRequestSpec covering: numeric-as-JSON-number (unchanged behavior), numeric-as-JSON-string (coerced), non-numeric string (400/JsonMappingException), empty string (None), whitespace-padded numeric string (coerced), and fractional numeric string (rejected).

Ran mvn -pl server -am test -DwildcardSuites=org.apache.livy.server.batch.CreateBatchRequestSpec,org.apache.livy.server.interactive.CreateInteractiveRequestSpec; all 14 tests pass.

Was this patch authored or co-authored using generative AI tooling?

Yes, this patch was co-authored using Claude Code (Anthropic).

[soumyadeeplogin]

Filed LIVY-1083 for this change.

[soumyadeeplogin]

Hi @gyogal / @roczei — could one of you approve the pending CI workflow run for this PR (fork-contributor approval gate)? Happy to address any review feedback as well. Thanks!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.10%. Comparing base (501e822) to head (015c296).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #558      +/-   ##
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+ Coverage     68.68%   73.10%   +4.41%     
- Complexity     1218     1295      +77     
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  Files           106      106              
  Lines          6815     6874      +59     
  Branches        836      855      +19     
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+ Hits           4681     5025     +344     
+ Misses         1666     1344     -322     
- Partials        468      505      +37     

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