[LIVY-1061] Add livy.server.app-name-prefix for interactive session YARN app names

[samuhasavak5]

What changes were proposed in this pull request?

This PR adds a new Livy server configuration property, livy.server.app-name-prefix, to allow cluster administrators to customize the default YARN application name for interactive Spark sessions.

Problem: When a client creates an interactive session via POST /sessions without specifying a name, Livy sets spark.app.name to the hardcoded value livy-session-. In HA or multi-cluster environments, this makes it difficult to distinguish Livy-managed applications in the YARN Resource Manager UI.

Solution:

Add LivyConf.SERVER_APP_NAME_PREFIX (livy.server.app-name-prefix, default: livy-session)
Use the configured prefix when building the default spark.app.name in InteractiveSession.create()
Document the new property in conf/livy.conf.template
Add unit tests for default and custom prefix behavior
Scope: Interactive sessions only. Batch sessions already support per-request naming via the REST API name field and are unchanged.

Backward compatibility: When the property is unset, behavior remains livy-session-.

Example:
livy.conf

livy.server.app-name-prefix = my-cluster

Creating a session without a name yields YARN application name: my-cluster-3

JIRA: LIVY-1061

How was this patch tested?

• Unit tests: Added tests in InteractiveSessionSpec for:
  -Default prefix (livy-session) when config is not set
  -Custom prefix (e.g. my-cluster) when livy.server.app-name-prefix is configured
• Build: mvn package -Pspark3 -Pscala-2.12 — livy-server and dependent modules built successfully
• Manual verification (recommended): Set livy.server.app-name-prefix in livy.conf, create an interactive session without name, and confirm the YARN RM UI shows -

No UI changes in this PR.

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

Yes, this was co-authored using Cursor to help generate the new test cases.

[samuhasavak5]

@roczei , @gyogal , @nileshrathi345 and @ArnavBalyan
could you please take a look when possible and review
thanks..!!!!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.63%. Comparing base (0efa2d7) to head (07db93b).
⚠️ Report is 30 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #534      +/-   ##
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+ Coverage     68.38%   68.63%   +0.25%     
- Complexity     1199     1215      +16     
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  Files           106      106              
  Lines          6711     6785      +74     
  Branches        831      834       +3     
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+ Hits           4589     4657      +68     
- Misses         1657     1660       +3     
- Partials        465      468       +3     

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[roczei]

@samuhasavak5

Thanks for this! Nice, focused change with good tests and docs. LGTM

[gyogal]

LGTM, thank you for your contribution @samuhasavak5 !