feat(sns): use NNS Governance for maturity modulation

[jasonz-dfinity]

Why

Mission 70 moved the maturity modulation computation from the CMC into NNS
Governance, which now exposes it via the new get_maturity_modulation query.
This switches SNS Governance to read maturity modulation from NNS Governance
instead of the CMC, so the legacy CMC-side flow can be deprecated.

What

• Add a NnsGovernanceClient trait + RealNnsGovernanceClient in
  rs/nervous_system/clients/, using bounded-wait ic_cdk calls. Modeled on
  the existing ExchangeRateCanisterClient.
• SNS Governance: replace the cmc field with nns_governance. The periodic
  update_maturity_modulation task now calls NNS Governance's
  get_maturity_modulation and stores current_value_permyriad (numerically
  equivalent to basis points) in SNS's existing current_basis_points field.
  SNS keeps the prior "skip update when no value" contract.
• NNS Governance: at canister init, seed heap_data.maturity_modulation with
  a neutral 0 permyriad value so callers get a usable response immediately
  rather than None while update_icp_xdr_rate_related_data accumulates
  enough XRC price history to compute a real value.
  updated_at_days_since_epoch is left None so the periodic task does not
  treat the placeholder as "already updated today".
• Test fixtures and call sites in SNS Governance migrate from
  MockCMC/FakeCmc to a new FakeNnsGovernanceClient.

Testing

• New unit test in get_maturity_modulation covers the init default.
• Updated update_icp_xdr_rate_related_data_tests assertions to reflect the
  new init default.
• The SNS integration test
  //rs/sns/integration_tests:neuron_test exercises the SNS to NNS Governance
  path end-to-end (including disburse_maturity and stake_maturity flows).

[Copilot]

Pull request overview

Switches SNS Governance from reading maturity modulation from the CMC to the new get_maturity_modulation query on NNS Governance. Adds a new client abstraction, migrates SNS Governance and its tests, and seeds NNS Governance with a neutral default at init so callers get a usable response immediately.

Changes:

• New NnsGovernanceClient trait, RealNnsGovernanceClient (bounded-wait ic_cdk call), and FakeNnsGovernanceClient in rs/nervous_system/clients/.
• SNS Governance replaces its cmc field with nns_governance; update_maturity_modulation now skips when NNS returns None, otherwise stores current_value_permyriad into current_basis_points.
• NNS Governance init seeds heap_data.maturity_modulation to a neutral Some(0) permyriad with updated_at_days_since_epoch = None; tests updated accordingly.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rs/nervous_system/clients/src/nns_governance_client.rs	New client module: trait, real impl, and test fake.
rs/nervous_system/clients/src/lib.rs	Expose new nns_governance_client module.
rs/nns/governance/src/heap_governance_data.rs	Seed maturity_modulation with neutral 0 permyriad at init.
rs/nns/governance/src/governance/tests/get_maturity_modulation.rs	Test renamed and updated for new init default.
rs/nns/governance/src/timer_tasks/update_icp_xdr_rate_related_data_tests.rs	Update assertions for new init default.
rs/sns/governance/canister/canister.rs	Wire RealNnsGovernanceClient(NNS_GOVERNANCE_CANISTER_ID) into Governance.
rs/sns/governance/src/governance.rs	Replace cmc: Box<dyn CMC> with nns_governance and use it in update_maturity_modulation.
rs/sns/governance/src/extensions.rs, proposal.rs, proposal/advance_sns_target_version.rs	Test setup migrated from MockCMC to FakeNnsGovernanceClient.
rs/sns/governance/src/governance/*_tests.rs	Test setups migrated from FakeCmc to FakeNnsGovernanceClient.
rs/sns/governance/tests/fixtures/mod.rs	Remove CmcFixture; switch GovernanceCanisterFixture to FakeNnsGovernanceClient.
rs/sns/governance/tests/governance.rs	Update fixture field reference.
rs/sns/integration_tests/src/neuron.rs	Migrate imports/comments and call sites to FakeNnsGovernanceClient.

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