misc: release old buffers before ensureCapacity growth

[lizhen-0710]

What problem does this PR solve?

Release the existing buffer before allocating a larger replacement in dwio::common::ensureCapacity(). The helper is used by Parquet decode buffers and other scan-side scratch buffers where growth can otherwise transiently hold both the old large allocation and the new larger allocation in the same memory pool.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 🚀 Performance improvement (optimization)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🔨 Refactoring (no logic changes)
• 🔧 Build/CI or Infrastructure changes
• 📝 Documentation only

Description

This reduces short-lived scan memory spikes without changing the returned buffer contents contract: callers already request capacity growth and do not depend on preserving old contents through this helper.

Add a regression test with a constrained memory pool. The test derives the old and new capacities from the allocator preferred size, then requests one byte beyond the old capacity so ensureCapacity must reallocate. Without releasing the old buffer first, the constrained pool cannot hold the old and new allocations at the same time and the test fails with MEM_CAP_EXCEEDED.

Performance Impact

• No Impact: This change does not affect the critical path (e.g., build system, doc, error handling).

• Positive Impact: I have run benchmarks.

  Click to view Benchmark Results

  Paste your google-benchmark or TPC-H results here.
  Before: 10.5s
  After:   8.2s  (+20%)
  

• Negative Impact: Explained below (e.g., trade-off for correctness).

Release Note

Please describe the changes in this PR

Release Note:

Release Note:
- Fixed a crash in `substr` when input is null.
- optimized `group by` performance by 20%.

Checklist (For Author)

• I have added/updated unit tests (ctest).
• I have verified the code with local build (Release/Debug).
• I have run clang-format / linters.
• (Optional) I have run Sanitizers (ASAN/TSAN) locally for complex C++ changes.
• No need to test or manual test.

Breaking Changes

• No

• Yes (Description: ...)

  Click to view Breaking Changes

  Breaking Changes:
  - Description of the breaking change.
  - Possible solutions or workarounds.
  - Any other relevant information.
  

[guhaiyan0221]

AlignedBuffer::allocate allocates more than 4M due to kPaddingSize, roughly equals to 1.5*4M. So ensureCapacity may not trigger date.reset and reallocate

[lizhen-0710]

Good catch, updated, thanks.

[guhaiyan0221]

LGTM!