feat(spice): fallback validators fetch missing contract code (6/6)

[darioush]

A non-designated validator that pulls a chunk's witness to endorse it for the all-stake fallback may need contract code the witness omits (the SPICE witness ships only the contract-accesses hash, not the bytecode). handle_spice_contract_code_request now admits a non-designated epoch validator's SpiceContractCodeRequest once the chunk is past the fallback window; designated validators may always request as before.

Adds a disable_compiled_contract_cache test-loop builder flag that gives every node a NoContractRuntimeCache, so validators must actually request contract code rather than reuse a precompiled copy. Without it the code-request gate is never exercised.

e2e: certifies-chunk-accessing-contract-code deploys a contract, calls it under a total designated-endorsement outage, and asserts the certified frontier reaches the call chunk via the fallback (which requires a cold non-designated validator to fetch the code).

[Copilot]

Pull request overview

This PR updates SPICE all-stake fallback behavior so that non-designated (but still epoch) validators can fetch missing contract bytecode needed to endorse chunks via the fallback once the chunk becomes fallback-eligible. It also extends the test-loop harness and adds an e2e-style test to ensure this path is exercised by forcing validators to fetch contract code rather than reuse precompiled artifacts.

Changes:

• Allow SpiceContractCodeRequest from non-designated epoch validators when fallback_eligible(...) is true (designated validators still allowed unconditionally).
• Add a disable_compiled_contract_cache flag to the test-loop builder/shared state to use NoContractRuntimeCache across nodes.
• Add a SPICE fallback test that deploys + calls a contract during a designated-endorsement outage and waits for the certified frontier to reach the call chunk.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test-loop-tests/src/tests/spice/all_stake_fallback.rs	Adds a new fallback certification test that requires cold contract code fetching, plus a small genesis-builder extension for adding user accounts.
test-loop-tests/src/setup/state.rs	Adds shared-state flag to disable the compiled contract cache across all nodes.
test-loop-tests/src/setup/setup.rs	Switches runtime contract cache between FilesystemContractRuntimeCache and NoContractRuntimeCache based on the new shared-state flag.
test-loop-tests/src/setup/builder.rs	Plumbs the new builder flag into SharedState and exposes a builder method to enable it.
chain/client/src/spice/data_distributor_actor.rs	Relaxes contract-code request gating to admit fallback-eligible requests from non-designated epoch validators.

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.64%. Comparing base (ed95ce9) to head (cb024bf).

Files with missing lines	Patch %	Lines
chain/client/src/spice/data_distributor_actor.rs	60.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #15942   +/-   ##
=======================================
  Coverage   73.64%   73.64%           
=======================================
  Files         865      865           
  Lines      191987   192004   +17     
  Branches   191987   192004   +17     
=======================================
+ Hits       141380   141408   +28     
+ Misses      46122    46106   -16     
- Partials     4485     4490    +5     

Flag	Coverage Δ
pytests-nightly	1.20% <0.00%> (-0.01%)	⬇️
unittests	70.01% <25.00%> (-0.02%)	⬇️
unittests-nightly	70.07% <25.00%> (+<0.01%)	⬆️
unittests-spice	65.99% <80.00%> (+0.03%)	⬆️

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[greptile-apps]

Greptile Summary

This PR allows authenticated non-designated epoch validators to request omitted contract code after a chunk enters the all-stake fallback window.

• Adds fallback-aware authorization for Spice contract-code requests.
• Adds a test-loop option that disables compiled-contract caching.
• Adds an end-to-end fallback test involving contract deployment and execution under a designated-endorsement outage.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Sequence Diagram

sequenceDiagram
    participant V as Non-designated epoch validator
    participant D as SpiceDataDistributorActor
    participant E as Epoch manager
    participant C as Spice core reader
    participant S as Contract storage
    V->>D: Signed contract-code request
    D->>E: Resolve validator and verify signature
    E-->>D: Authenticated epoch validator
    D->>C: Check chunk fallback eligibility
    C-->>D: Eligible after fallback window
    D->>S: Load requested contract code
    S-->>D: Contract bytecode
    D-->>V: SpiceContractCodeResponse

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