Seed the checkpoint scope-key hasher for deterministic restore

[PratikDhanave]

Summary

Checkpoint.UnmarshalJSON (workflow/internal/checkpoint/checkpoint_json.go) rebuilds StateData with scopeKeyHasherImpl, whose Hash used a zero-value maphash.Hash:

func (scopeKeyHasherImpl) Hash(s workflow.ScopeKey) uint64 {
	var h maphash.Hash   // zero value → new RANDOM seed on first use
	s.Hash(&h)
	return h.Sum64()
}

A zero-value maphash.Hash selects a fresh random seed on first use, so the same ScopeKey hashes to a different value on every call. On the hashmap.Map produced by unmarshal this means:

• Load/Delete/overwrite break — the value stored during unmarshal is hashed with one seed, and a later Load re-hashes with a different seed and misses.
• Shared-scope keys no longer collapse — two keys that are Equal (shared scope ignores the executor ID) get different hashes and are kept as separate entries, so StateManager.ImportState re-imports both and which value survives a restore is non-deterministic.

The sibling hasher in workflow/internal/execution/state.go already seeds with a fixed process-wide theSeed; the checkpoint copy just forgot to.

Fix

Add a fixed process-wide scopeKeyHashSeed = maphash.MakeSeed() and h.SetSeed(scopeKeyHashSeed) in Hash, mirroring state.go.

Public API

No exported symbols change.

Tests

Adds TestCheckpoint_JsonRoundtrip_StateDataRemainsLoadable (black-box, in json_test.go): round-trips a Checkpoint through JSON and asserts its StateData is still Load-able. It fails before this change (Load returns false) and passes after. (Existing tests missed it because they only consume restored StateData via .All() iteration, which is seed-independent.)

[Copilot]

Pull request overview

This PR fixes a determinism bug in checkpoint JSON restore by ensuring the ScopeKey hasher used to rebuild Checkpoint.StateData is seeded consistently, so restored state remains usable via Load/Delete and key-collapsing behavior works as intended.

Changes:

• Seed the checkpoint-local ScopeKey hasher with a process-wide maphash.Seed during hashing.
• Add a regression test that round-trips a Checkpoint JSON payload and asserts restored StateData.Load(key) succeeds.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
workflow/internal/checkpoint/checkpoint_json.go	Seed the ScopeKey hasher used during JSON unmarshal so hashing is consistent across calls within the process.
workflow/internal/checkpoint/json_test.go	Add a test that validates restored StateData remains Load-able after JSON unmarshal.

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