feat(spice): non-tracking validators pull witness for fallback (4/6)

[darioush]

Lets a non-designated epoch validator that doesn't track a chunk's shard contribute to the all-stake fallback. Once a chunk is past the fallback window and still uncertified, the data distributor pulls the chunk's witness (start_waiting_on_fallback_witness) so the validator can apply and endorse it; the existing witness-validation flow then broadcasts the endorsement, which the fallback tally certifies.

contribute_fallback_endorsements runs per processed block: for each uncertified, fallback-eligible chunk the node is a non-designated epoch validator of, it pulls the witness if it isn't already tracking the shard or hasn't already endorsed.

Adds SpiceChunkEndorsement::shard_id() and the test infra used by the fallback e2e tests: a DesignatedSpiceEndorsements drop condition (drops endorsements from designated senders so chunks can certify only via the fallback) and a last_certified_block_header node helper.

e2e: certifies-without-designated-endorsements (total designated outage) and certifies-across-epoch-boundary.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 98 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.61%. Comparing base (5b84f99) to head (c2c9b00).

Files with missing lines	Patch %	Lines
chain/client/src/spice/data_distributor_actor.rs	0.00%	59 Missing ⚠️
test-loop-tests/src/utils/network.rs	0.00%	20 Missing ⚠️
test-loop-tests/src/setup/drop_condition.rs	0.00%	10 Missing ⚠️
test-loop-tests/src/utils/node.rs	0.00%	5 Missing ⚠️
core/primitives/src/spice/chunk_endorsement.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@                              Coverage Diff                              @@
##           darioush/spice/all-stake-fallback-certify   #15940      +/-   ##
=============================================================================
- Coverage                                      69.64%   69.61%   -0.04%     
=============================================================================
  Files                                            947      947              
  Lines                                         204472   204570      +98     
  Branches                                      204472   204570      +98     
=============================================================================
- Hits                                          142406   142404       -2     
- Misses                                         57318    57416      +98     
- Partials                                        4748     4750       +2     

Flag	Coverage Δ
pytests-nightly	1.10% <0.00%> (-0.01%)	⬇️
unittests	69.35% <0.00%> (-0.04%)	⬇️
unittests-nightly	69.30% <0.00%> (-0.04%)	⬇️

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[Copilot]

Pull request overview

Enables SPICE all-stake fallback certification to progress even when a validator is not designated for a chunk and does not track the chunk’s shard, by having the data distributor proactively pull the chunk’s witness once it becomes fallback-eligible. Adds supporting test-loop infrastructure and end-to-end fallback tests that simulate a full designated-endorser outage and an epoch-boundary transition.

Changes:

• Add per-processed-block logic in SpiceDataDistributorActor to request fallback-eligible witnesses for non-designated, non-tracking epoch validators.
• Extend SPICE endorsement primitives with SpiceChunkEndorsement::shard_id() and add test-loop helpers for observing certification progress and selectively dropping designated endorsements.
• Add new test-loop SPICE e2e tests covering fallback-only certification and certification across an epoch boundary.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test-loop-tests/src/utils/node.rs	Adds last_certified_block_header() helper for fallback e2e assertions.
test-loop-tests/src/utils/network.rs	Adds a network dropper that filters designated SPICE endorsements to force fallback-only certification in tests.
test-loop-tests/src/tests/spice/mod.rs	Registers the new all_stake_fallback test module.
test-loop-tests/src/tests/spice/all_stake_fallback.rs	New e2e tests for fallback-only certification and across-epoch-boundary certification.
test-loop-tests/src/setup/drop_condition.rs	Adds DropCondition::DesignatedSpiceEndorsements wiring to install the new dropper.
core/primitives/src/spice/chunk_endorsement.rs	Adds SpiceChunkEndorsement::shard_id() accessor used by tests/infrastructure.
chain/client/src/spice/data_distributor_actor.rs	Implements contribute_fallback_endorsements and witness-pull initiation for fallback-eligible uncertified chunks.

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