LVBS: Enable safe TA page table teardown

[Sangho Lee (sangho2)]

This PR enables safe TA page table teardown for LVBS/OP-TEE. It aligns the LVBS platform's page table teardown with the Arc-based ownership model, deferring page table destruction until the final owner drops it. Although this teardown strategy is unsafe in the general case (e.g., COW/shared mappings), it is safe for the LVBS/OP-TEE runner because it strictly controls session/instance lifecycles and does not share any TA page frames.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/inherent_method_missing.ron

Failed in:
  LinuxKernel::delete_task_page_table, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/e4cb07139d283354e0d9980cffee4790ce599f04/litebox_platform_lvbs/src/lib.rs:746
  PageTableManager::delete_task_page_table, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/e4cb07139d283354e0d9980cffee4790ce599f04/litebox_platform_lvbs/src/lib.rs:388

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/inherent_method_missing.ron

Failed in:
  OpteeShim::release_user_mappings, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/e4cb07139d283354e0d9980cffee4790ce599f04/litebox_shim_optee/src/lib.rs:381