src: support V8 sandbox memory cage in allocators

[codebytere]

When V8_ENABLE_SANDBOX is enabled, all ArrayBuffer backing stores must be allocated within the V8 memory cage — external pointers cannot be directly wrapped and must be copied into V8-managed memory instead. This commit refactors allocators in node_buffer.cc, node_serdes.cc, and node_trace_events.cc to route allocations through ArrayBuffer::Allocator::NewDefaultAllocator() when the sandbox is enabled, ensuring memory lands inside the cage. In node_serdes.cc, ValueSerializer::Delegate is also extended with ReallocateBufferMemory/FreeBufferMemory overrides so the serializer's internal buffer is cage-allocated from the start.

Tested by making the following change:

diff --git a/configure.py b/configure.py
index fa47e9c4854..f4342d2c718 100755
--- a/configure.py
+++ b/configure.py
@@ -2047,7 +2047,7 @@ def configure_v8(o, configs):
   # Until we manage to get rid of all those, v8_enable_sandbox cannot be used.
   # Note that enabling pointer compression without enabling sandbox is unsupported by V8,
   # so this can be broken at any time.
-  o['variables']['v8_enable_sandbox'] = 0
+  o['variables']['v8_enable_sandbox'] = 1 if options.enable_pointer_compression else 0
   # We set v8_enable_pointer_compression_shared_cage to 0 always, even when
   # pointer compression is enabled so that we don't accidentally enable shared
   # cage mode when pointer compression is on.

and running with ./configure --ninja --experimental-enable-pointer-compression

This allows Electron to reduce/remove a patch.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.66%. Comparing base (da5843b) to head (78e9555).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_serdes.cc	84.61%	0 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62237      +/-   ##
==========================================
- Coverage   89.67%   89.66%   -0.02%     
==========================================
  Files         676      676              
  Lines      206469   206503      +34     
  Branches    39537    39543       +6     
==========================================
+ Hits       185157   185161       +4     
- Misses      13448    13457       +9     
- Partials     7864     7885      +21     

Files with missing lines	Coverage Δ
src/api/environment.cc	78.92% <ø> (ø)
src/node_buffer.cc	67.91% <ø> (-0.10%)	⬇️
src/node_trace_events.cc	82.07% <100.00%> (ø)
src/node_serdes.cc	70.23% <84.61%> (+1.69%)	⬆️

... and 27 files with indirect coverage changes

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