ci(java): add Java coverage baseline to post-merge workflow

[hubcio]

Codecov needs a base report from master for accurate PR diffs.
Add java-coverage job to post-merge.yml mirroring the Rust
pattern, rename coverage to rust-coverage, and configure
flag_management in codecov.yml with carryforward enabled.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.03%. Comparing base (5306a91) to head (6021b61).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2707   +/-   ##
=======================================
  Coverage   69.03%   69.03%           
=======================================
  Files         571      571           
  Lines       55525    55525           
  Branches    55525    55525           
=======================================
+ Hits        38332    38334    +2     
+ Misses      15318    15316    -2     
  Partials     1875     1875           

Flag	Coverage Δ
rust	69.03% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 5 files with indirect coverage changes

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