parity: extend LCOW HCS document parity tests with permutations, gaps, and error paths

Add lcow_doc_permutations_test.go with three new test functions that build

on the existing TestLCOWDocumentParity / TestLCOWSandboxOptionsFieldParity

scaffolding:

  * TestLCOWDocumentParityPermutations exercises 26 annotation and option

    permutations covering CPU partials, memory edge cases (overcommit,

    cold discard, deferred commit, non-256-aligned size), boot modes

    (UEFI vs kernel direct, initrd vs VHD), feature flags (scratch

    encryption, writable shares, policy-based routing, writable overlay),

    HvSocket extra ports, kernel arg combinations (VPCI, time sync,

    process dump, dump dir), shim option overrides, and regression cases

    for previously-known nil-vs-empty struct gaps in CpuGroup and

    StorageQoS.

  * TestLCOWErrorPathParity asserts that both pipelines fail (or both

    succeed) on invalid boot files paths and on the deferred commit +

    physically backed memory conflict.

  * TestLCOWSandboxOptionsFieldParityNonDefault verifies SandboxOptions

    field parity (NoWritableFileShares, EnableScratchEncryption,

    PolicyBasedRouting, FullyPhysicallyBacked, VPMEMMultiMapping) when

    callers opt into non-default values, complementing the existing

    default-config test.

All cases force VPMemCount=0 on the legacy side because the v2 builder

rejects vPMem entirely (commit da11cd2), so the documents are directly

comparable. vPMem-specific permutations and inverted-assertion gap tests

from earlier revisions of this PR are dropped.

Signed-off-by: Shreyansh Sancheti <shsancheti@microsoft.com>

Author: Shreyansh Sancheti