fix(qwen3.5): the capture #1054 dropped was not redundant, and MSVC says so

[localai-bot]

e34d71379 removed [&kRequired] from the refuse lambda in
CheckMoeQuantLayoutSupported as "the redundant namespace-scope capture". Two
different variables carry that name, and only one of them is namespace-scope:
kMoeExpertLayoutHelp (:894) is a static const std::string and needs no
capture, while kRequired (:929) is a function-local const std::string&
bound to it, is odr-used in the lambda body, and therefore must be captured.
MSVC rejects the result with error C3493 and main has not compiled under it
since.

Naming the namespace-scope constant inside the lambda satisfies both compilers
at once: MSVC has nothing left to capture, and the AppleClang diagnostic #1054
set out to remove stays removed. Reverting to [&kRequired] would fix MSVC and
reintroduce that diagnostic. Behavior is unchanged because kRequired IS
kMoeExpertLayoutHelp; the reference is still used by the other refusal at
:947, so it does not become dead.

Verified by CI rather than locally for the compiler that matters: this box has
no MSVC, so windows-msvc-cpu and windows-msvc-vulkan on this PR are the
gate. g++ -std=c++20 -fsyntax-only on the TU returns 0.

Worth recording why this landed green. The guarding gate is a source-TEXT
assertion -- it "rejects const auto refuse = [&kRequired] and finds
const auto refuse = []" -- which passes whether or not the translation unit
compiles, so it cannot fail on the defect it was written to guard. #1054 also
records that its host had neither CMake nor Clang, so the change was compiled by
neither compiler it concerns. And windows-msvc-* are skipped on main (#503),
so no baseline existed for the break to regress from and it first surfaced on an
unrelated PR (#983).

An instance of #503, not a new report of it.

Fixes #1068

documentation-checkpoint refused an earlier revision of this commit, because
FEATURE_SURFACE_PREFIXES covered all of src/vllm/model_executor/models/ and
so any edit there owed docs/FEATURES.md -- for a change that adds no
capability and alters no behavior. That demand is exactly what #1054 answered
with prose, and that prose is what crossed the check-public-doc-tables budgets
and blocked every push in the repository (#1055). Rather than feed it again,
the trigger was repaired: 8fa405bb7 (#1086, issue #595) now keys
feature_surface off a change to the set of REGISTER_VLLM_MODEL(...)
registrations. This commit changes none, so the gate passes it on its own terms
and no exception is claimed.

FOLLOWING_AGENTS_PROTOCOL

Following-Agents-Protocol: true
AI-Assisted: true
Assisted-by: ClaudeCode:claude-opus-5 [ClaudeCode]