Resolve uSID container ownership for zero-node locators

[watal]

Description

Fix uSID container ownership resolution for zero-node locators.

Type of change

• New feature
• Bug fix
• Refactoring
• Documentation
• Build / CI

How was this tested?

make ci

Checklist

• make ci passes locally
• Tests added or updated if needed
• Documentation updated if needed

[Copilot]

🟢 Approval recommended

The logic change is small, directly addresses the reported bug, and is covered by updated and newly added targeted tests for the relevant edge cases.

Pull request overview

Fixes SRv6 uSID/C-SID container ownership resolution when the matched locator has zero node bits (e.g., a /32 “locator-only” advertisement), so such SIDs resolve directly to the locator owner instead of being treated as non-authoritative and falling back to adjacency-SID validation.

Changes:

• Update SIDIndex.usidContainerOwner to treat nodeBits<=0 locators as a successful match that resolves to the matched locator owner.
• Adjust ValidateExplicitPath test expectation to accept node-bits=0 locator ownership resolution (no adjacency-SID error).
• Add internal tests covering node-bits=0 behavior, including single-owner, conflicting-owner, declared-structure, and nested-locator scenarios.

File summaries

File	Description
pkg/table/sid_validate.go	Changes uSID container ownership resolution for node-bits=0 locators to resolve directly to the locator owner.
pkg/table/sid_validate_test.go	Updates explicit-path validation test to expect success when traversing a node-bits=0 locator-owned uSID container.
pkg/table/sid_validate_internal_test.go	Adds focused unit tests for node-bits=0 locator ownership semantics and edge cases.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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