refactor(rust): enforce style guide rules for pub/module visibility

[chet]

This enables the unreachable-pub lint for every Rust package, enforcing
the module visibility rules introduced in
#4522, and is the last PR following #4593, #4600, #4630, #4635, #4647, #4660, #4661, #4662, #4663, #4664, #4665, and #4682.

Default-feature builds exposed a small set of declarations that
all-feature builds kept reachable through test and benchmark APIs. This
scopes those declarations to their callers while keeping those
feature-gated APIs public only when their features are enabled.

Primary callouts are:

• Add -D unreachable-pub beside -D warnings in the shared Cargo
  compiler flags so packages outside the workspace lint table are covered.
• Narrow the remaining API Core, BMC mock, Health, NVLink manager, and
  carbide-lints declarations to the visibility their callers require.
• Keep the Health benchmark and NVLink test-support APIs public only when
  their features are enabled, without adding lint allowances.
• Update the custom-lint fixture to compile under the same visibility
  rule it helps enforce.
• Leave runtime behavior, wire contracts, and generated source unchanged.

Related issues

This supports #4558.

This supports the updated Rust visibility scoping guidelines in
STYLE_GUIDE.md, established in
#4522.

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Breaking Changes

• This PR contains breaking changes

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

Additional Notes

The workspace now contains 109 Rust packages, including 17 that do not
inherit the root workspace lint table. Configuring the compiler lint in
.cargo/config.toml covers those packages without adding a custom lint or
changing their manifests.

No new allow(unreachable_pub) exceptions are introduced. The existing
exceptions remain limited to the generated-code and ABI boundaries reviewed
in #4682. The Health benchmark
and NVLink test-support APIs instead use feature-specific visibility so their
public surface only exists in the configuration that exports it.

Local CodeRabbit and read-only Claude reviews completed before the final fix
round. The applicable Health default-build finding is incorporated; the final
feature-boundary changes also received a fresh read-only review.

[coderabbitai]

Summary by CodeRabbit

• Refactor
  • Tightened internal API visibility across services, health monitoring, hardware mocks, and developer tooling without changing runtime behavior.
  • Improved separation between production, test, and benchmark interfaces.
• Bug Fixes
  • Updated service validation tests to use the default configured chart version.
• Tests
  • Refined compiler diagnostic fixtures and test-only builders for clearer access boundaries.
• Chores
  • Enabled stricter Rust compiler linting to catch unintentionally exposed APIs.

Walkthrough

The PR enables the workspace-wide unreachable-pub lint and narrows internal APIs across API-core, health, NVLink, BMC mock, and lint crates. Test and benchmark interfaces retain scoped access where required.

Changes

Internal API visibility

Layer / File(s)	Summary
Lint policy and API-core visibility .cargo/config.toml, crates/api-core/src/*	The workspace denies unreachable-pub. API-core constants, types, methods, and test builders use restricted visibility. Xplane tests use the default service configuration.
Mock and health API boundaries crates/bmc-mock/src/hw/*, crates/health/src/sink/*	The Lenovo rack mock and health event mappers use scoped visibility. OTLP constructors separate production, test, and benchmark access.
Switch certificate monitor execution paths crates/nvlink-manager/src/switch_cert_monitor.rs	Production and test-support monitor APIs are separated. Shared construction and iteration logic injects result creation.
Carbide lint implementation visibility lints/carbide-lints/src/*	Internal lint statics, types, methods, and helpers use crate or private visibility.
Lint fixture visibility expectations lints/carbide-lints/tests/fixtures/app/src/*	The fixture narrows transaction and async method visibility. The expected diagnostic reflects the private declaration.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• NVIDIA/infra-controller#4665: Narrows Rust visibility in overlapping health event-mapper code.
• NVIDIA/infra-controller#4682: Addresses Rust visibility and unreachable_pub linting.
• NVIDIA/infra-controller#4571: Enables related Rust visibility lint enforcement and fixes affected code.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the primary change: enforcing Rust visibility rules through the unreachable-pub lint.
Description check	✅ Passed	The description directly explains the lint enablement, visibility changes, feature boundaries, testing, and unchanged runtime behavior.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

_{Comment @coderabbitai help to get the list of available commands.}

[chet]

@coderabbitai full_review, thanks!

[coderabbitai]

@chet I will perform a complete new review of pull request #4743.

🐇🔍

✅ Action performed

Full review finished.