Fix live Java checks and Temurin discovery

[e-kotov]

Summary

Harden Temurin version discovery by validating the summary endpoint response and confirming majors against downloadable Temurin assets.
Rework rJava version checks to use a shared subprocess Java environment, prefer callr when available, and keep an Rscript fallback with the same env contract.
Update the live-test workflow and test suite so package behavior, subprocess isolation, and Temurin fallback paths are covered consistently.

Verification

Run Rscript -e 'devtools::test(stop_on_failure = FALSE)'
Expected local result: [ FAIL 0 | WARN 0 | SKIP 61 | PASS 519 ]

[codecov]

Codecov Report

❌ Patch coverage is 90.82969% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.53%. Comparing base (a6bd250) to head (2c78499).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
R/java_env.R	68.96%	18 Missing ⚠️
R/internal_utilities.R	96.07%	2 Missing ⚠️
R/java_valid_versions.R	99.12%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
+ Coverage   89.12%   89.53%   +0.41%     
==========================================
  Files          27       27              
  Lines        2776     2887     +111     
==========================================
+ Hits         2474     2585     +111     
  Misses        302      302              

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