refactor: measure splitTime in doSplit and shuffleWriteTime at operator level

[HaoChen-ch]

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

1. Measure splitTime directly. Replace the subtraction with a dedicated NanosecondTimer splitTimer(&splitTime_) scoped to the real split work inside doSplit() (buildPartition2Row + preAlloc + splitRowVector). finalizeMetrics() now simply assigns metrics_.splitTime= splitTime_.

2. Measure shuffleWriteTime at the operator level. Move it into SparkShuffleWriter (ShuffleWriterNode), wrapping addInput (split), reclaim (spill) and noMoreInput→stop. It now reflects the full wall time of the shuffle-write operator (init + addInput + reclaim + stop) instead of just split + stop.

3. Narrow the flattenTime scope. Tighten the NanosecondTimer(&flattenTime_) blocks so they cover only the flatten calls (ensureFlatten / ensurePartialFlatten / ensureVectorLoaded), excluding the subsequent split/evict logic.

4. Remove the now-unused totalSplitTime_ and stopTime_ members and their timers in split() / stop() across all three writers.

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.