test(epoch-manager): seed ChunkProducers in test harnesses + missing-row invariant

[stedfn]

Follow-up to #15908 (now merged to master); rebased onto master. Raises test fidelity for the EarlyKickout DB-anchored chunk-producer path before PR6 (#15843) makes DB[anchor] != sample.

• Add test-only EpochManager::seed_chunk_producers_for_test, mirroring production's save_chunk_producers_for_header, and seed DBCol::ChunkProducers at the nightly test recording sites (epoch-manager helpers, shard_tracker, tests/mod.rs, runtime/tests.rs, garbage_collection) so the aggregator exercises the real DB-read path instead of always hitting the sampler fallback.
• Gate the seed helper's EarlyKickout check on the anchor's own epoch (get_epoch_info(block_info.epoch_id())), matching production. The epoch-after gate over-seeds dead rows for last-of-epoch anchors across an activation edge; those rows are never read by the aggregator/witness cross-epoch arm.
• In anchored_chunk_producers_for_aggregator, split the miss branch: debug_assert that a missing same-epoch row only happens for the epoch-sync-installed first block, and debug_assert!(false) for the unreachable account-not-in-epoch case.
• Fix the miss-branch fallback to sample at anchor.height + CHUNK_GRANDPARENT_ANCHOR_HEIGHT_OFFSET (the producer the writer would have stored) instead of the chunk height; identical for consecutive heights, exact under skips.
• Tests: test_aggregator_missing_epoch_first_block_row_falls_back, test_aggregator_skip_anchor_uses_anchor_height. Full near-epoch-manager nightly suite 106/0.

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.59%. Comparing base (715e0f9) to head (a839b7a).

Files with missing lines	Patch %	Lines
chain/epoch-manager/src/lib.rs	78.84%	10 Missing and 1 partial ⚠️
chain/chain/src/runtime/tests.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15944      +/-   ##
==========================================
- Coverage   72.60%   72.59%   -0.01%     
==========================================
  Files         952      952              
  Lines      205109   205163      +54     
  Branches   205109   205163      +54     
==========================================
+ Hits       148914   148939      +25     
- Misses      51218    51243      +25     
- Partials     4977     4981       +4     

Flag	Coverage Δ
pytests-nightly	1.10% <0.00%> (-0.01%)	⬇️
unittests	69.53% <91.66%> (-0.02%)	⬇️
unittests-nightly	69.54% <79.31%> (+<0.01%)	⬆️
unittests-spice	66.96% <79.31%> (-0.03%)	⬇️

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