test(state-sync): early kickout coverage for synced nodes

[stedfn]

Nightly-gated integration coverage that synced nodes do not stall under EarlyKickout.

EarlyKickout resolves chunk producers from a grandparent anchor backed by DBCol::ChunkProducers. A same-epoch anchor with a missing row returns ChunkProducerNotInDB and aborts chunk-signature / witness verification. These tests check that a node joining via the sync pipeline holds the rows its catch-up reads depend on.

• test_early_kickout_far_horizon_observer: full epoch -> header -> state -> block sync of an observer; probes that the same-epoch grandparent-anchor row the consensus read uses is present and that the read agrees with the row.
• test_early_kickout_state_sync_shuffling: a shuffling validator state-syncs a newly assigned shard and keeps producing; asserts state sync actually ran and that the witness path records no miss_db_entry.

Green proves syncs are safe because header and block processing seed the rows. The missing-row path is unreachable on master today: header sync seeds the contiguous range and DBCol::ChunkProducers is not garbage collected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.27%. Comparing base (b8a041c) to head (164eaa9).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15983      +/-   ##
==========================================
+ Coverage   73.19%   73.27%   +0.07%     
==========================================
  Files         856      857       +1     
  Lines      188394   188902     +508     
  Branches   188394   188902     +508     
==========================================
+ Hits       137904   138417     +513     
- Misses      46070    46071       +1     
+ Partials     4420     4414       -6     

Flag	Coverage Δ
pytests-nightly	1.22% <ø> (-0.01%)	⬇️
unittests	69.91% <ø> (+0.05%)	⬆️
unittests-nightly	69.92% <ø> (+0.06%)	⬆️
unittests-spice	65.54% <ø> (+0.08%)	⬆️

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