feat: Extract shared ExchangeRateCanisterClient into ic-nervous-system-clients

[jasonz-dfinity]

Why

CMC and the upcoming Governance timer task (next PR) both need XRC-calling logic. This extracts the shared code to avoid duplication.

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

What

Moved exchange-rate-canister calling logic from CMC into a new shared module in ic-nervous-system-clients:

• New exchange_rate_canister_client.rs with ExchangeRateCanisterClient trait, RealExchangeRateCanisterClient, error types, validation helpers, and shared constants
• The trait adds timestamp: Option so callers can request historical rates; CMC always passes None
• CMC exchange_rate_canister.rs is updated to use the shared types; its local duplicates are removed

This is a pure refactor -- no behavior change to CMC.

PR Chain

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

Testing

Existing CMC tests cover the refactor.

[Copilot]

Pull request overview

Refactors Exchange Rate Canister (XRC) calling logic out of the Cycles Minting Canister (CMC) into the shared ic-nervous-system-clients crate so multiple NNS components can reuse a single implementation.

Changes:

• Added exchange_rate_canister_client module to ic-nervous-system-clients (trait + real client + shared constants + validation/conversion helpers).
• Updated CMC to use the shared XRC client/trait and removed local duplicate XRC-calling code.
• Wired new dependencies in Cargo/Bazel manifests (and lockfile) for the new shared module.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
rs/nns/cmc/src/main.rs	Switches RealExchangeRateCanisterClient import to the shared clients crate.
rs/nns/cmc/src/exchange_rate_canister.rs	Replaces local XRC trait/impl/validation with shared ExchangeRateCanisterClient + validate_exchange_rate; updates mocks accordingly.
rs/nns/cmc/Cargo.toml	Adds dependency on ic-nervous-system-clients.
rs/nns/cmc/BUILD.bazel	Adds Bazel dependency on //rs/nervous_system/clients.
rs/nervous_system/clients/src/lib.rs	Exposes the new exchange_rate_canister_client module.
rs/nervous_system/clients/src/exchange_rate_canister_client.rs	Introduces shared XRC client, errors, validation, and permyriad conversion helper.
rs/nervous_system/clients/Cargo.toml	Adds ic-cdk + ic-xrc-types dependencies for the new client implementation.
rs/nervous_system/clients/BUILD.bazel	Adds Bazel deps for ic-cdk and ic-xrc-types.
Cargo.lock	Records new crate dependency edges.

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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

1. unreleased_changelog.md: not updated, and not required — this PR is a pure code-move refactor (extracting the XRC client out of CMC into ic-nervous-system-clients) with no observable behavior change from CMC's perspective.
2. BREAKING changes: none — the move is internal; CMC still calls the same logic.
3. Data migration: none — no persisted canister state changes.
4. Security review: not needed — existing logic moves between crates without modification.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 9 changed files in this pull request and generated 2 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.