fix(proto): stop wrapping capabilities in a process submessage

[G4614]

Fixes a real production incident: restarting an existing, non-privileged box failed with "unknown Linux capability ''" because #646 wrapped capabilities in a new ProcessOptions submessage on the same wire field number every already-deployed 0.9.8+ guest still expects a flat ContainerCapabilities on.

Test plan:

• make clippy
• make fmt:check:rust
• make test:unit:rust (1025 + 49 passed)

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claude exited 1

stdout:
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stderr:
<empty>

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📝 Walkthrough

Walkthrough

Container capabilities now use a top-level field instead of nested process options. Runtime resolution tracks privileged-mode ownership, and the portal, guest service, guest initialization, and Python SDK use the flattened representation.

Changes

Container capability security flow

Layer / File(s)	Summary
Wire contract flattening src/shared/proto/boxlite/v1/service.proto	ContainerAdvancedOptions exposes capabilities directly at field 1. The obsolete ProcessOptions message was removed.
Runtime capability resolution src/boxlite/src/runtime/advanced_options.rs, src/boxlite/src/runtime/options.rs	Runtime resolution stores capabilities directly. set_capabilities replaces the policy and clears privileged-mode ownership tracking. Tests cover capability withdrawal and preservation of caller-provided policies.
Portal, guest, and SDK propagation src/boxlite/src/portal/interfaces/container.rs, src/boxlite/src/litebox/init/tasks/guest_init.rs, src/guest/src/service/container.rs, sdks/python/src/options.rs	Portal initialization sends direct capabilities. Guest capability handling and version gating read the direct field. Python SDK conversion uses set_capabilities. Wire transmission tests verify the configured capability.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🔵 Low · up to 92a8d

The change prevents explicit capability policies from being cleared accidentally and restores compatibility with existing guests, but the protobuf field-shape change may affect consumers built against the intermediate nested format. The PR is mergeable with explicit owner awareness and follow-up validation for all schema consumers.

Sequence Diagram(s)

sequenceDiagram
  participant AdvancedBoxOptions
  participant ContainerInterface
  participant GuestContainerService
  participant GuestInit
  AdvancedBoxOptions->>ContainerInterface: provide top-level capabilities
  ContainerInterface->>GuestContainerService: transmit capabilities at field 1
  GuestContainerService->>GuestInit: provide advanced capabilities
  GuestInit->>GuestInit: apply capability version gate

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Possibly related issues

• boxlite-ai/boxlite#1295: Adds set_capabilities to keep privileged-mode ownership tracking synchronized, while documenting the remaining limitation of direct field assignment.

Possibly related PRs

• boxlite-ai/boxlite#1156: Modifies privileged-mode capability ownership and withdrawal behavior in the same runtime area.
• boxlite-ai/boxlite#1204: Updates the same capability configuration and guest initialization paths.
• boxlite-ai/boxlite#1047: Introduces the capability-policy implementation and related security configuration paths.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the incident and test results but omits the required Summary, Call graph, Changes, and Risks / rollout sections.	Use the repository template and add the before-and-after call graph, notable changes, verification steps, and any applicable rollout risks.

✅ Passed checks (4 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 and concisely identifies the primary protocol change: removing the process submessage around capabilities.

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Actionable comments posted: 1

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Inline comments:
In `@src/boxlite/src/runtime/advanced_options.rs`:
- Around line 765-773: Update set_privileged and its related capability-handling
logic to track whether the canonical privileged capability policy was installed
by set_privileged(true), rather than inferring ownership solely from the
capability shape. On a true-to-false transition, reset capabilities only when
that provenance is recorded, then clear the provenance appropriately; preserve
independently configured add=["ALL"] policies. Add a regression test covering an
externally configured ALL policy through a true → false cycle.

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Actionable comments posted: 1

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Inline comments:
In `@src/shared/proto/boxlite/v1/service.proto`:
- Around line 391-400: Resolve the field-1 type change for
ContainerAdvancedOptions.capabilities by preserving the previously released
ProcessOptions schema contract; do not replace it with ContainerCapabilities
under the same tag. If ProcessOptions was definitively never released, use the
project’s reviewed Buf breaking-change exception mechanism instead, with
appropriate justification.

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• src/boxlite/src/litebox/init/tasks/guest_init.rs
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• src/boxlite/src/runtime/options.rs
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Actionable comments posted: 1

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Inline comments:
In `@src/boxlite/src/runtime/advanced_options.rs`:
- Around line 785-790: Ensure every replacement of
AdvancedBoxOptions.capabilities clears capabilities_installed_by_privileged,
preventing set_privileged(false) from removing caller-owned policies; introduce
an encapsulated setter or equivalent API and update capability writes to use it.
Add a regression test covering set_privileged(true), replacing capabilities with
an add=["ALL"] policy, then set_privileged(false), verifying the caller policy
remains intact.

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