fix: Use end_lsn for commit fragment offset to fix max_batch_size boundary bug

[alco]

Summary

Fixes a bug where transactions with exactly max_batch_size (default: 100) changes had their commit fragment silently dropped by ShapeLogCollector.

Root Cause

When a transaction has exactly max_batch_size changes:

1. The 100th change triggers maybe_flush/1, returning Fragment 1 with last_log_offset = (final_lsn, 198)
2. maybe_flush creates a new empty fragment but did NOT reset state.last_log_offset
3. When the Commit arrives, Fragment 2 gets the same last_log_offset = (final_lsn, 198)
4. ShapeLogCollector.handle_txn_fragment/2 drops Fragment 2 as duplicate (offset <= last_processed_offset)

Solution

Use Commit.end_lsn instead of Begin.final_lsn for the commit fragment's last_log_offset. In Postgres logical replication, end_lsn > final_lsn, guaranteeing the commit fragment's offset is always greater than preceding data fragments.

Changes

• message_converter.ex: Remove last_log_offset from state; use end_lsn for commit offset
• replication_client.ex: Make max_batch_size configurable via NimbleOptions
• Unit tests: Update expected offsets and add boundary case test
• Integration tests: New end-to-end tests for 99, 100, 101, and 200 row transactions

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.36%. Comparing base (4c7a4b6) to head (25e45e0).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3764   +/-   ##
=======================================
  Coverage   87.36%   87.36%           
=======================================
  Files          23       23           
  Lines        2011     2011           
  Branches      531      531           
=======================================
  Hits         1757     1757           
  Misses        252      252           
  Partials        2        2           

Flag	Coverage Δ
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (ø)
packages/start	82.83% <ø> (ø)
packages/typescript-client	93.47% <ø> (ø)
packages/y-electric	56.05% <ø> (ø)
typescript	87.36% <ø> (ø)
unit-tests	87.36% <ø> (ø)

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[blacksmith-sh]

Found 2 test failures on Blacksmith runners:

Failures

Test	View Logs
Elixir.Electric.ShapeCacheTest/test get_or_create_shape_handle/ 2 against real db crashes when initial snapshot query fails to return data quickly enou gh	View Logs
Elixir.Electric.Shapes.ConsumerTest/ test transaction handling with real storage should terminate after :hibernate_after ms	View Logs