[Sandbox] Make sqlUnifiedQueryVersion overridable; force mavenLocal precedence

[mch2]

Description

• test-ppl-frontend/build.gradle: sqlUnifiedQueryVersion now reads -PsqlUnifiedQueryVersion=… with default 3.7.0.0-SNAPSHOT. No build.gradle edit needed for local SQL plugin checkouts.
• test-ppl-frontend + analytics-engine build.gradle: force mavenLocal to repo position 0. Declaring mavenLocal() first isn't enough — plugin chains prepend remote repos before our block runs, so we re-insert: def local = mavenLocal(); remove(local); add(0, local). Fresh feature-branch jars now win over stale ci.opensearch.org SNAPSHOTs.
• analytics-engine: drop redundant testImplementation for unified-query-{api,core,ppl} — already pulled in transitively via the test-ppl-frontend project dep.

Same intent as the reverted #21578 (rest-test plugin zips bundle the fresh feature-branch unified-query), but doesn't break publish: empty ~/.m2/ on the publish runner makes add(0, mavenLocal) a no-op there, falling through to OS Snapshots — unlike #21578's excludeGroup, which left publish with nothing to resolve from.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

✅ Gradle check result for 07679be: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.51%. Comparing base (36809cc) to head (07679be).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #21589      +/-   ##
============================================
+ Coverage     73.50%   73.51%   +0.01%     
+ Complexity    74644    74599      -45     
============================================
  Files          5980     5980              
  Lines        338777   338777              
  Branches      48848    48848              
============================================
+ Hits         249011   249054      +43     
+ Misses        69946    69854      -92     
- Partials      19820    19869      +49     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.