feat: Offline write batch support for IcebergSource

[ntkathole]

What this PR does / why we need it:

When a FeatureView's batch_source is an IcebergSource, calling fs.write_to_offline_store() or fs.push(to=PushMode.ONLINE_AND_OFFLINE) was not supported.

Added Offline write batch support for IcebergSource.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 9.09091% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.07%. Comparing base (b5090c6) to head (bb7905e).

Files with missing lines	Patch %	Lines
sdk/python/feast/infra/offline_stores/duckdb.py	8.69%	21 Missing ⚠️
...ffline_stores/contrib/spark_offline_store/spark.py	10.00%	9 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6829      +/-   ##
==========================================
- Coverage   47.09%   47.07%   -0.03%     
==========================================
  Files         419      419              
  Lines       51885    51918      +33     
  Branches     7528     7534       +6     
==========================================
+ Hits        24437    24440       +3     
- Misses      25699    25729      +30     
  Partials     1749     1749              

Flag	Coverage Δ
go-feature-server	30.58% <ø> (ø)
python-unit	48.38% <9.09%> (-0.03%)	⬇️

Files with missing lines	Coverage Δ
...ffline_stores/contrib/spark_offline_store/spark.py	35.63% <10.00%> (-0.37%)	⬇️
sdk/python/feast/infra/offline_stores/duckdb.py	35.27% <8.69%> (-1.92%)	⬇️

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b5090c6...bb7905e. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.