fix: make WasmBase::doAfterVmCallActions reentry-safe (drain-to-local)

[johnlanni]

Problem

WasmBase::doAfterVmCallActions() drains its after_vm_call_actions_ queue with an
unguarded while (!after_vm_call_actions_.empty()) loop, popping and running one action
at a time:

while (!self->after_vm_call_actions_.empty()) {
  auto f = std::move(self->after_vm_call_actions_.front());
  self->after_vm_call_actions_.pop_front();
  f();
}

If an action re-enqueues itself (or otherwise adds a new action) during the drain,
the freshly-added action is observed by the same loop and executed again in the same
frame. Under synchronous reentry this loop never sees an empty queue and spins forever,
pinning a worker at 100% CPU.

This is reachable from a plugin today. A host call made from within an after-VM-call
action re-enters the VM synchronously, and that nested VM call's DeferAfterCallActions
guard calls doAfterVmCallActions() again. For example, in the Envoy host, a deferred
onRedisCall/onHttpCall failure callback that calls
injectEncodedDataToFilterChain (or sendLocalReply) re-enters the VM and schedules
another after-VM-call action; the outer loop keeps observing a non-empty queue and never
returns.

The same shared-queue reentry is also the mechanism behind #326: an action queued in one
phase (e.g. continueStream() from onRequestHeaders) is drained by a nested
doAfterVmCallActions() belonging to a different phase (e.g. onResponseHeaders
triggered by sendLocalReply), so the action "escapes" into the wrong phase.

Fix

Swap the member queue into a local std::deque before draining, then iterate the
snapshot exactly once:

std::deque<std::function<void()>> local;
local.swap(self->after_vm_call_actions_);
for (auto &f : local) {
  f();
}

Actions re-added by a callback during the drain land in the (now-empty) member queue and
are therefore picked up by the next-outer DeferAfterCallActions frame, instead of being
re-run in this loop. This:

• Breaks the synchronous-reentry spin (bounded work per call).
• Prevents cross-phase action escape (the re-added action runs in the outer frame, not
  inside a nested VM call of another phase) — see addAfterVmCallAction may cause unexpected problems #326.
• Preserves ordering semantics for legitimately deferred work; re-added actions are still
  executed, just deferred to the next-outer frame.

It is a header-only change to a small inline method, zero-ABI and transparent to plugins
(no signature or behavior change visible to Wasm modules).

Test

Adds TEST_P(TestVm, DoAfterVmCallActionsReentrySafe) in test/wasm_test.cc, wired into
the existing //test:wasm_test target. It registers a self-re-enqueuing action and
asserts the count advances by exactly one per doAfterVmCallActions() call. On the old
code this test hangs forever (documenting the regression); with the fix each drain runs
the snapshot once and defers the re-added copy.

Refs

• Fixes the reentry mechanism described in addAfterVmCallAction may cause unexpected problems #326.
• Downstream context / real-world repro: Wasm 插件死循环：onRedisCallFailure + injectEncodedDataToFilterChain 嵌套导致 Envoy CPU 100% 跑满 higress-group/higress#4034 (worker CPU spin via a
  deferred Redis-failure callback that injects into the filter chain).