feat(server): add integration test for message deduplication

[seokjin0414]

Summary

Closes #2872

• Add integration test for the message deduplication pipeline (7-step scenario)
• Fix server panic when all messages in a batch are duplicates
• Fix partition offset calculation after dedup removes mid-batch messages
• Fix deduplicator not being created for lazily-initialized partitions

Bug fixes

Empty batch panic (messages.rs): After prepare_for_persistence() removes all duplicate messages, subsequent .unwrap() calls on first_timestamp(), last_timestamp(), last_offset() panic. Added empty batch guard.

Offset calculation (messages.rs): last_offset was computed as current_offset + count - 1, which doesn't account for offset gaps created by dedup removal. Changed to use segment.end_offset (the actual last offset from batch).

Deduplicator not created (partitions.rs): init_partition_inner() hardcoded None for the message_deduplicator parameter. Added create_message_deduplicator() call matching the bootstrap path.

Integration test scenario

Step	Description	Validates
1	Send 10 messages with id=0 (auto UUID)	All pass through with unique IDs
2	Send 10 messages with explicit IDs 1-10	Normal dedup registration
3	Re-send IDs 1-10 with different payload	Duplicates rejected, original payload preserved
4	Send all-duplicate batch	No server crash, count unchanged
5	Send mixed batch (IDs 6-15)	Only new IDs 11-15 accepted
6	Verify offsets	Monotonically increasing after dedup
7	Wait for TTL expiry, re-send IDs 1-10	Previously seen IDs accepted again

Test plan

• cargo fmt --all -- --check
• cargo clippy -p server -p integration --all-targets -- -D warnings
• cargo test -p integration --test mod -- message_deduplication (CI)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.74%. Comparing base (27f0f11) to head (46db1ea).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3099      +/-   ##
============================================
+ Coverage     72.71%   72.74%   +0.02%     
  Complexity      943      943              
============================================
  Files          1117     1117              
  Lines         96285    96288       +3     
  Branches      73485    73506      +21     
============================================
+ Hits          70014    70045      +31     
+ Misses        23725    23669      -56     
- Partials       2546     2574      +28     

Components	Coverage Δ
Rust Core	73.53% <100.00%> (+0.07%)	⬆️
Java SDK	62.30% <ø> (ø)
C# SDK	69.11% <ø> (-0.29%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.40% <ø> (-0.13%)	⬇️
Go SDK	38.97% <ø> (ø)

Files with missing lines	Coverage Δ
core/server/src/shard/system/messages.rs	88.00% <100.00%> (+0.02%)	⬆️
core/server/src/shard/system/partitions.rs	78.51% <100.00%> (+0.07%)	⬆️
core/server/src/streaming/partitions/journal.rs	85.54% <100.00%> (+0.17%)	⬆️

... and 27 files with indirect coverage changes

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