feat: multi-tenant deployment support

[alexm-redhat]

Summary

Adds tenant-scoped ownership enforcement to the OpenShell gateway, enabling multi-tenant sandbox and provider isolation.

Core ownership engine (auth/ownership.rs, 448 lines):

• OwnershipGuard trait with check_owner() and stamp_owner() methods
• Tenant extraction from OIDC sub claims with configurable claim field
• Owner-stamped sandbox creation and owner-filtered list/get/delete
• Owner-stamped provider CRUD with (owner, name) uniqueness

Gateway integration:

• OIDC tenant extraction in auth/oidc.rs
• RBAC guard augmented with ownership checks in auth/authz.rs
• Tenant-scoped sandbox gRPC handlers in grpc/sandbox.rs
• Tenant-scoped provider gRPC handlers in grpc/provider.rs
• Compute driver tenant labels on sandbox containers
• CLI --multi-tenant flag and multi_tenant.enabled config toggle (off by default)

21 files changed, +825/-17 lines across openshell-core and openshell-server.

Test plan

• cargo build — all crates compile
• cargo test — 3,072 passed, 0 failed, 10 ignored
• Unit tests for ownership guard enforcement (stamp, check, filter)
• Unit tests for tenant-scoped sandbox and provider gRPC handlers
• Integration tests for OIDC tenant extraction
• Manual E2E: deploy with multi_tenant.enabled: true, create sandboxes as two different OIDC users, verify isolation

Addresses #1722

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[alexm-redhat]

I have read the DCO document and I hereby sign the DCO.

[alexm-redhat]

recheck