feat(governance): expose get_maturity_modulation query endpoint

[jasonz-dfinity]

Why

PR #9847 computes maturity modulation locally but does not expose it via the canister API.
This adds a query endpoint so that external tools and dashboards can read the current
maturity modulation value, as defined by Mission 70.

https://dfinity.atlassian.net/browse/NNS1-4319

What

• Add MaturityModulation, GetMaturityModulationRequest, and GetMaturityModulationResponse types to the ic-nns-governance-api crate
• Add Governance::get_maturity_modulation() method
• Add #[query] fn get_maturity_modulation endpoint to the governance canister
• Add corresponding types and service entry to governance.did
• Add pb::api From conversion for MaturityModulation

PR Chain

• ⬅️ Previous: feat(governance): derive Mission 70 maturity modulation from XRC price history #9847

[Copilot]

Pull request overview

This PR exposes the Mission 70 “maturity modulation” value through the Governance canister’s public Candid interface so external tools/dashboards can query it.

Changes:

• Added MaturityModulation, GetMaturityModulationRequest, and GetMaturityModulationResponse to ic-nns-governance-api.
• Added Governance::get_maturity_modulation() and a corresponding #[query] get_maturity_modulation canister endpoint.
• Updated governance.did and added the required pb→api conversion for MaturityModulation.

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
rs/nns/governance/src/pb/conversions/mod.rs	Adds pb→api From conversion for MaturityModulation.
rs/nns/governance/src/governance.rs	Adds a getter that exposes maturity modulation from heap state via API response type.
rs/nns/governance/canister/governance.did	Declares new request/response + MaturityModulation types and adds the new service method.
rs/nns/governance/canister/canister.rs	Implements the new public query endpoint that returns the modulation value.
rs/nns/governance/api/src/types.rs	Introduces new public API types for the query endpoint and payload.

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