refactor(nns): drop inner Options on MaturityModulation

[jasonz-dfinity]

Why

MaturityModulation's two scalar fields were optional, generating
Option<i32> / Option<u64> in prost. Both inner Options were
redundant: current_value_permyriad had no meaningful absent state
(only ever set, never read as None with distinct semantics), and
updated_at_days_since_epoch already had 0 available as a "never
measured" sentinel — current_day is always >> 0 in practice, so a
real measurement can never collide.

What

• Drop optional from MaturityModulation in governance.proto;
  regenerate the prost bindings so the fields are bare i32 / u64.
• initialize_governance continues to pre-seed
  Some(MaturityModulation { current_value_permyriad: 0, updated_at_days_since_epoch: 0 })
  so spawning and disbursement keep their pre-PR behavior at init.
• update_maturity_modulation keys the "first calculation, no
  speed-limit baseline" branch on updated_at_days_since_epoch == 0.
• pb→api conversion maps updated_at_days_since_epoch == 0 back to
  None so the public API contract is preserved.

Testing

The wire-format safety of dropping optional is demonstrated by a
review-time scaffold commit on a side branch:
7b92bde
— it defines parallel prost messages with/without optional and
round-trips encoded bytes through the other shape, asserting
Some(v) ↔ v and the default-elision asymmetry (None ↔ 0,
Some(0) → 0) in both directions. Not merged here because the
OldMaturityModulation shape no longer exists in the tree.

[github-actions]

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   non-breaking).

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[Copilot]

Pull request overview

This PR removes redundant inner Option wrappers from the protobuf MaturityModulation message by dropping optional on its scalar fields, switching the prost bindings to plain i32/u64. The implementation is updated to use updated_at_days_since_epoch == 0 as the “never measured” sentinel, while preserving the public API behavior by mapping that sentinel back to None in pb→api conversion.

Changes:

• Update MaturityModulation protobuf schema to use non-optional scalar fields and regenerate Rust prost bindings.
• Update maturity modulation computation logic to use updated_at_days_since_epoch == 0 as the “no baseline yet” indicator.
• Adjust call sites and tests to the new non-Option fields and preserve API semantics in conversion code.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rs/nns/governance/tests/governance.rs	Update test helper to construct MaturityModulation with scalar fields.
rs/nns/governance/src/timer_tasks/update_icp_xdr_rate_related_data.rs	Switch update logic and logging to scalar fields; use 0 sentinel for “never measured”.
rs/nns/governance/src/timer_tasks/update_icp_xdr_rate_related_data_tests.rs	Update task tests to the scalar-field representation and sentinel behavior.
rs/nns/governance/src/pb/conversions/mod.rs	Map updated_at_days_since_epoch == 0 to None for the public API; wrap scalar permyriad into Some.
rs/nns/governance/src/heap_governance_data.rs	Seed maturity_modulation with { current_value_permyriad: 0, updated_at_days_since_epoch: 0 } at init.
rs/nns/governance/src/governance/tla/mod.rs	Adjust extraction of permyriad from MaturityModulation now that it’s scalar.
rs/nns/governance/src/governance/tests/get_maturity_modulation.rs	Update API-facing tests to expect sentinel-driven updated_at mapping.
rs/nns/governance/src/governance/disburse_maturity.rs	Adjust modulation value read path to scalar field.
rs/nns/governance/src/governance/disburse_maturity_tests.rs	Update test construction to scalar current_value_permyriad.
rs/nns/governance/src/governance.rs	Adjust modulation value read path to scalar field.
rs/nns/governance/src/gen/ic_nns_governance.pb.v1.rs	Regenerated prost bindings reflecting non-optional fields and sentinel documentation.
rs/nns/governance/proto/ic_nns_governance/pb/v1/governance.proto	Remove optional from MaturityModulation scalar fields and document 0 sentinel.

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