refactor(nns): Replace pb_api with api

[jasonz-dfinity]

The Governance API types are no longer pb, so it shouldn't be called pb_api. This PR is simply s/pb_api/api/.

[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?

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Reference(s)

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GOVERNANCE_CHECKLIST_REMINDER_DEDUP

No behavior change - trivial renaming

[Copilot]

Pull request overview

This pull request performs a straightforward refactoring to rename the pb_api alias to api throughout the codebase. The change is motivated by the fact that the Governance API types are no longer protocol buffer (pb) types, making the old name misleading. This is a pure rename operation with no functional changes.

Key Changes

• Renamed the import alias from use ic_nns_governance_api as pb_api to use ic_nns_governance_api as api
• Updated all references to pb_api:: to use api:: instead
• Updated function names containing pb_api to use api (e.g., convert_guest_launch_measurements_from_pb_to_pb_api → convert_guest_launch_measurements_from_pb_to_api)

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
rs/nns/governance/src/pb/proposal_conversions.rs	Updated import alias and all type references from pb_api to api throughout conversion functions
rs/nns/governance/src/pb/conversions/tests.rs	Updated test code to use api instead of pb_api
rs/nns/governance/src/pb/conversions/mod.rs	Extensive updates to conversion implementations and function names to use api instead of pb_api
rs/nns/governance/src/governance/tests/neurons_fund.rs	Updated test code to use api instead of pb_api

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