fix(machine-validation): skip reboot when disabled

[iExalt]

Description

NICO currently enters the machine-validation reboot path even when machine validation is disabled, issuing an unnecessary host restart and increasing startup latency.

This change:

• skips disabled machine validation directly from RebootHost, before issuing a host reboot
• reuses one completion path for disabled validation from both RebootHost and MachineValidating
• preserves the existing wait when validation is disabled while a reboot is already in flight
• preserves the upstream guard for validation runs that are no longer active
• makes site-explorer fixtures branch on the observed machine state instead of the validation configuration
• adds regression coverage for both disabled-validation paths

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.)

Related Issues (Optional)

Related to #2486 feat: skip machine validation reboot when validation is disabled

Breaking Changes

• N/A

Testing

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

Validation performed against current NVIDIA/infra-controller:main:

• git diff --check
• cargo fmt --all -- --check
• cargo test -p carbide-machine-controller (19 passed)
• cargo test -p carbide-api-core --no-default-features --no-run

Additional Notes

• N/A

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Walkthrough

This PR refactors machine validation skip behavior when disabled: introduces a reusable skip helper that marks validations as skipped and transitions hosts appropriately, integrates it into the validation state handler, updates test infrastructure to handle the new terminal state, and verifies via regression tests that reboot invariance is preserved.

Changes

Machine Validation Skip Behavior

Layer / File(s)	Summary
Skip validation helper crates/machine-controller/src/handler/machine_validation.rs	New skip_machine_validation async helper implements the skip workflow: marks the validation Skipped, updates last_machine_validation_list metrics, and transitions the host to HostInit::Discovered { skip_reboot_wait: true } via transactional DB mutation. Supports both handler paths via a single, auditable implementation.
Handler integration of skip logic crates/machine-controller/src/handler/machine_validation.rs	Handler now delegates skip behavior to the helper rather than inlining it. In RebootHost arm, when machine validation is disabled, the handler early-returns the skip helper result. In MachineValidating arm, the previous inline skip block is replaced with a helper call, reducing duplication.
Test fixture state handling crates/api-core/src/tests/common/api_fixtures/site_explorer.rs	Test fixtures host_state_controller_iterations and host_state_controller_iterations_with_machine_validation now recognize HostInit::Discovered { skip_reboot_wait: true } as a terminal state. Removes unused reboot_completed import, extends stop-condition logic, and refactors branching to distinguish between validation-completion vs. skip paths and route post-validation state transitions accordingly.
Test cases for skip behavior crates/api-core/src/tests/machine_validation.rs	Adds MachineStateHandlerBuilder import. Updates test_machine_validation_disabled to snapshot last_reboot_requested before and after controller iteration, asserting invariance. Introduces test_machine_validation_disabled_waits_for_in_flight_reboot to verify that disabling validation mid-flight suppresses reboot requests and transitions the validation to "skipped" completion.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

The PR exhibits moderate-to-high complexity due to: (1) dense state-machine logic in handler and fixture refactoring, (2) heterogeneous changes across three files with interdependencies, (3) new test infrastructure requiring careful inspection of state branching and assertions, and (4) the need to verify that skip semantics correctly suppress reboot behavior and mark validation status without regression. The implementation is clean and avoids duplication, but the state transitions and test harness updates warrant thorough verification.

Possibly related issues

• #2486: This PR directly implements the requested behavior of marking machine validation runs as skipped and transitioning hosts to HostInit::Discovered { skip_reboot_wait: true } without requesting a reboot when machine validation is disabled, fulfilling the feature requirements.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the primary change: skipping reboot execution when machine validation is disabled, which directly addresses the core issue of unnecessary host restarts.
Description check	✅ Passed	The description is comprehensive and directly related to the changeset, covering the problem statement, implementation approach, test coverage, and validation steps.
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.

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[ajf]

@sunilkumar-nvidia please review

[ajf]

/ok to test aad6829

[ajf]

fyi @iExalt GitHub can't verify your signed commits belong to you, they show as unverified.

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pull-request-2488.docs.buildwithfern.com/infra-controller

[sunilkumar-nvidia]

Thanks for the PR, this avoids unnecessary reboots when machine validation is disabled and should save time in skipped-validation flows.

[iExalt]

fyi @iExalt GitHub can't verify your signed commits belong to you, they show as unverified.

this is a problem when using GH to rebase, let me rebase locally