[UR][L0] Disable USM residency P2P restriction by default (#22796)
Disable the "Restrict USM residency to peers with enabled P2P access"
feature by default: P2P access is now treated as enabled for all
connectable peer devices out of the box, so USM allocations are made
resident on all of them without requiring an explicit
ext_oneapi_enable_peer_access() call. When the restriction is disabled,
urUsmP2P(Enable|Disable)PeerAccessExp become no-ops that always return
success, matching the behavior of the Level Zero V1 adapter.
Add the SYCL_UR_L0_RESTRICT_USM_RESIDENCY_TO_P2P environment variable
to opt back into the restrictive behavior on demand, requiring P2P
access to be explicitly enabled for a peer before USM allocations
become resident on it, and enforcing the enable/disable state machine.
Update the tests that exercise this feature
(sycl/test-e2e/USM/P2P/p2p_usm_residency.cpp and
unified-runtime/test/adapters/level_zero/v2/memory_residency.cpp) to
set the new environment variable so they keep testing the restrictive
behavior. Document the new variable in EnvironmentVariables.md.
Ref: #21889
Ref: MLSL-4536
Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>
