fix: serialize ByteBuffer remaining view

[DarrenChangJR]

What this PR does and why

Fastjson2 currently serializes heap-backed ByteBuffer values by returning the
entire backing array. This ignores the buffer's position, limit, and
arrayOffset, so a window exposing [1,2,3] from [9,1,2,3,8] is serialized
and parsed as all five bytes.

Direct and read-only buffers already serialize only their remaining view. This
change makes heap buffers follow the same rule:

• retain the zero-copy fast path for a full backing-array view;
• copy only arrayOffset + position .. arrayOffset + limit for heap windows
  and slices;
• use a duplicate for buffers without an accessible array, avoiding temporary
  mutation and restoration of the source position.

Tests

The expanded ByteBufferTest covers:

• heap buffers with nonzero position and a smaller limit;
• heap slices with a nonzero backing-array offset;
• equivalent direct and read-only views;
• text JSON and JSONB round trips;
• preservation of source position and limit;
• existing full-array and Base64 behavior.

Validation performed:

• focused ByteBufferTest;
• mvn -pl core -DskipTests validate;
• full mvn -pl core test suite: 7,982 tests, no failures or errors;
• external Fastjson 1.2.83 / Fastjson2 compatibility matrix with equivalent
  heap, slice, and direct views.

Checklist

• Heap, slice, direct, and read-only buffer implementations are covered.
• Text JSON and JSONB paths are covered.
• The common full-array path remains allocation-free.
• No public API change is required.

中文说明

修改内容和原因

Fastjson2 当前对有底层数组的 ByteBuffer 直接返回完整 array()，忽略
position、limit 和 arrayOffset。因此，从 [9,1,2,3,8] 中仅暴露
[1,2,3] 的窗口，会被错误地序列化并反序列化为全部五个字节。

Direct 和只读 buffer 已经只序列化 remaining view。本修改统一该语义：完整底层
数组仍走零拷贝快速路径；heap window 和 slice 只复制有效区间；没有可访问数组的
buffer 通过 duplicate 读取，避免临时修改原对象的 position。

测试

测试覆盖非零 position/较小 limit 的 heap buffer、带非零 array offset 的
slice、等价的 direct 和只读 view、文本 JSON 与 JSONB 往返，以及源 position
和 limit 不变。定向测试、Maven validate 和完整 7,982 个 core 测试均通过。