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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[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

How to Satisfy This Automatic Review

1. Go to the bottom of the pull request page.

2. Look for where it says this bot is requesting changes.

3. Click the three dots to the right.

4. Select "Dismiss review".

5. In the text entry box, respond to each of the numbered items in the previous
   section, declare one of the following:

• Done.

• $REASON_WHY_NO_NEED. E.g. for unreleased_changelog.md, "No
  canister behavior changes.", or for item 2, "Existing APIs
  behave as before.".

Brief Guide to "Externally Visible" Changes

"Externally visible behavior change" is very often due to some NEW canister API.

Changes to EXISTING APIs are more likely to be "breaking".

If these changes are breaking, make sure that clients know how to migrate, how to
maintain their continuity of operations.

If your changes are behind a feature flag, then, do NOT add entrie(s) to
unreleased_changelog.md in this PR! But rather, add entrie(s) later, in the PR
that enables these changes in production.

Reference(s)

For a more comprehensive checklist, see here.

GOVERNANCE_CHECKLIST_REMINDER_DEDUP