Improve NonRelocatable vector annotations and move-assignment state

[4og]

Description

Follow up on the review feedback from PR #599 for the NonRelocatable vector API.

Review the remaining methods and add [[nodiscard]] where ignoring a return value is likely unintended. Update move-assignment behavior so that a moved-from vector has both its size and capacity reset to zero.

Acceptance criteria

• Applicable NonRelocatable vector methods use [[nodiscard]] consistently with surrounding container APIs.
• Following move assignment, the source vector reports size() == 0 and capacity() == 0. (⚠️ Check if it make sense to reset the capacity)
• Tests cover the moved-from object's size and capacity after move assignment.

References

• PR #599 review comment by @mihajlo-k.