fix: decode index metadata only from manifest v6

[yuruiz]

Summary

• decode raw MILV index metadata only when manifest version is at least 6
• preserve Avro encoding for legacy ColumnGroupFile properties
• add coverage for raw manifest v6 index metadata and legacy v1-v3 behavior

Validation

• ColumnGroupsTest.LegacyV*: 4/4 passed
• ColumnGroupsTest.Index*: 3/3 passed
• container build: passed
• git diff --check: passed

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yuruiz
To complete the pull request process, please assign shaoting-huang after the PR has been reviewed.
You can assign the PR to them by writing /assign @shaoting-huang in a comment when ready.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.41%. Comparing base (dac5781) to head (92a34b3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #621   +/-   ##
=======================================
  Coverage   76.41%   76.41%           
=======================================
  Files         173      173           
  Lines       17561    17551   -10     
  Branches     2655     2653    -2     
=======================================
- Hits        13419    13412    -7     
+ Misses       4142     4139    -3     

Flag	Coverage Δ
cpp	79.00% <100.00%> (+<0.01%)	⬆️
python	44.45% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 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.

[jiaqizho]

/lgtm