Fix remote fetch and pin cleanup, and tighten test isolation

[mkhazraee]

Fix resource cleanup when remote fetch completion races with timeout or framework pin release fails.

• Release discarded fetch slots when a successful completion arrives after main-thread expiry, preserving the failed result.
• Retry failed framework pin releases during polling. Remove descriptors from usable state when release starts, and defer overlapping acquisitions until release succeeds or their original deadline expires.
• Apply the same release handling to late and invalid pin results.
• Give Guard integration tests explicit ZMQ context teardown and remove enforced test ordering.
• Make the shared fake NIXL agent reject mismatched descriptor counts and sizes.
• Avoid unused _BlockRecord allocations and repeated prefix slicing.
• Document deferred eviction-heap growth with a TODO.

Regression coverage includes fetch completion timing and slot reuse, pin-release failures, blocked reacquisition and expiry, and discarded pin results.

[karya0]

LGTM