feat(nns): Reduce maximum dissolve delay to 2 years and clamp all neurons

[jasonz-dfinity]

Why

As part of Mission 70, we want to reduce the maximum dissolve delay from 8 years to 2 years. To maintain the invariant that all neuron dissolve delays are at or below the maximum, existing neurons must be clamped.

https://dfinity.atlassian.net/browse/NNS1-4313

What

• Replace the MAX_DISSOLVE_DELAY_SECONDS constant with a max_dissolve_delay_seconds() function that returns 2 years when the mission-70 flag is enabled, 8 years otherwise
• Clamp all neuron dissolve delays to the new maximum in post_upgrade, guarded by the flag
• Record each neuron pre-clamp dissolve state in a new neuron_id_to_pre_clamp_dissolve_state Governance field for recovery purposes (the snapshot doubles as an idempotency guard)
• Add NeuronDissolveStateSnapshot protobuf message and plumb it through heap governance data
• Adjust test values to use the dynamic maximum

Testing

Unit tests added for clamp_dissolve_delay and clamp_dissolve_delay_for_all_neurons_or_panic. Existing tests updated to use the dynamic max.

[Copilot]

Pull request overview

This PR reduces the NNS Governance maximum neuron dissolve delay from 8 years to 2 years behind the Mission 70 flag, and adds an upgrade-time migration that clamps all existing neurons to the new maximum while recording a per-neuron pre-clamp snapshot for potential recovery.

Changes:

• Introduce max_dissolve_delay_seconds() (2y when Mission 70 is enabled, otherwise 8y) and update logic/tests to use it.
• Add post-upgrade clamping of all neurons’ dissolve states, storing pre-clamp snapshots in new persisted governance state.
• Extend protobuf + heap governance plumbing to persist NeuronDissolveStateSnapshot, and add/adjust unit tests accordingly.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
rs/nns/governance/src/governance.rs	Adds dynamic max dissolve delay function/constants; clamps all neurons on upgrade and stores snapshots.
rs/nns/governance/src/governance/create_neuron.rs	Validates requested dissolve delay against the dynamic maximum and updates error formatting.
rs/nns/governance/src/neuron/dissolve_state_and_age.rs	Replaces static cap with dynamic max and adds clamp_dissolve_delay helper.
rs/nns/governance/src/neuron/types.rs	Updates voting power dissolve-delay bonus denominator to dynamic max; adds clamping on AbridgedNeuron.
rs/nns/governance/src/storage/neurons.rs	Adds stable-store iteration to clamp dissolve delay for all neurons and return snapshots.
rs/nns/governance/src/neuron_store.rs	Exposes a NeuronStore wrapper method to clamp all neurons and return snapshots.
rs/nns/governance/src/heap_governance_data.rs	Persists the new snapshot map through heap governance split/reassemble.
rs/nns/governance/proto/ic_nns_governance/pb/v1/governance.proto	Adds NeuronDissolveStateSnapshot and governance map field.
rs/nns/governance/src/gen/ic_nns_governance.pb.v1.rs	Regenerated Rust protobuf bindings for the new messages/fields.
rs/nns/governance/tests/governance.rs	Updates tests to use the dynamic maximum and adjust expectations under Mission 70.
rs/nns/governance/tests/merge_neurons.rs	Updates proptest ranges to use max_dissolve_delay_seconds().
rs/nns/governance/src/voting.rs	Updates tally expectations in tests to account for Mission 70 dissolve-delay bonus changes.
rs/nns/governance/src/neuron/types/tests.rs	Updates dissolve-delay-related tests to use the dynamic maximum.
rs/nns/governance/src/storage/neurons/neurons_tests.rs	Updates eight-year-gang tests to use the pre-mission constant.
rs/nns/governance/src/neuron_store/neuron_store_tests.rs	Adds unit tests for clamping all neurons + snapshot correctness.
rs/nns/governance/src/neuron_store/metrics/tests.rs	Updates metrics tests to be compatible with the dynamic max.
rs/nns/governance/src/governance/tests/mod.rs	Updates tests using max dissolve delay to reference the pre-mission constant.

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