fix: guard ThrottleMultiplier against congestion tier-count changes

[AlexKehayov]

Description

ThrottleMultiplier.resetCongestionLevelStarts cloned the state-restored congestionLevelStarts array without checking its length against the active congestion config. updateMultiplier then indexes that array over activeConfig.multipliers().length tiers. If the number of congestion tiers (fees.percentCongestionMultipliers) changed between when the array was saved and when it is restored — and the restored array is shorter — the deterministic
fee/handle path throws ArrayIndexOutOfBoundsException on the next updateMultiplier after a restart/reconnect, crashing the node.

(The init ordering makes this reachable: resetExpectations() sizes the array to the current tier count via rebuildState(), then syncFromCongestionLevelStarts() overwrites it with the saved array, and ensureConfigUpToDate() returns false so rebuildState() is skipped before the out-of-bounds read.)

This reconciles the restored array against the active tier count: on a mismatch it logs a warning and skips the restore (discarding stale warm-up history) so the multiplier re-warms from a correctly-sized array instead of indexing out of bounds.

Behavior

• No change in the normal case. When the number of tiers is unchanged, the restored array length matches the active tier count and it is cloned exactly as before.
• The guard runs only when the restored array length differs from the active tier count — the case that crashes today. It then drops the stale start times and lets the multiplier re-accumulate from the default (1x) over minCongestionPeriod. Both inputs (the saved array length and the configured tier count) are identical on every node, so the guard behaves the same across the network and stays deterministic.
• Not externally triggerable, and not a consensus/funds change. The tier count comes only from fees.percentCongestionMultipliers — node-local startup config, not a @NetworkProperty, and not writable by any transaction. The only way to change the number of tiers is an operator/local-config or shipped-default change that takes effect on the next restart. Without this check, adding a tier to the shipped default would crash the handle path on every node at the next restart.

Testing

• Added ThrottleMultiplierTest.testResetCongestionLevelStartsIgnoresTierCountMismatch: with a 3-tier active config, restoring a 2-element congestionLevelStarts array is ignored (the array stays sized to the active tier count) and the next updateMultiplier does not throw. The test fails against the previous code (expected: <3> but was: <2>, then an ArrayIndexOutOfBoundsException on update) and passes with this change.

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[lfdt-bot]

✅ Snyk checks have passed. No issues have been found so far.

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...a/node/app/fees/congestion/ThrottleMultiplier.java	80.00%	0 Missing and 1 partial ⚠️

@@             Coverage Diff              @@
##               main   #26760      +/-   ##
============================================
- Coverage     70.47%   70.47%   -0.01%     
+ Complexity    11673    11672       -1     
============================================
  Files          2579     2579              
  Lines        108277   108282       +5     
  Branches      12100    12100              
============================================
- Hits          76313    76308       -5     
- Misses        27986    27993       +7     
- Partials       3978     3981       +3     

Files with missing lines	Coverage Δ	Complexity Δ
...a/node/app/fees/congestion/ThrottleMultiplier.java	92.92% <80.00%> (-0.60%)	0.00 <0.00> (ø)

... and 3 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 24bc7f7
▶️ Tests: 21769 executed
⚪️ Checks: 48/48 completed

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