GH-49315: [C++] Optimize Decimal128 abs; add exec-only TPC-H Q1 benchmark

[Manas103]

Rationale for this change

Profiling BM_Tpch_Q1_ExecOnly in Acero shows a hotspot in BasicDecimal128::Abs(), and the existing TPC‑H Q1 benchmark mixes data generation with execution, making it hard to isolate execution costs. This change introduces an exec‑only Q1 benchmark for cleaner profiling and removes an avoidable temporary in Abs() to reduce overhead in decimal‑heavy execution. It also adjusts TDigest test tolerances on MSVC to align with existing libc++ precision behavior.

What changes are included in the PR?

• Add BM_Tpch_Q1_ExecOnly that pre‑generates the lineitem table and benchmarks only plan execution
• Refactor Q1 setup to build an exec plan from a pre‑generated table source
• Optimize BasicDecimal128::Abs() to use IsNegative() directly (avoids temporary + operator<)
• Relax TDigest test tolerance on MSVC to match libc++ behavior

Are these changes tested?

Yes. I ran the C++ unit test subset labeled unittest using CTest with 4-way parallelism and failure output enabled:

• ctest -j4 -L unittest --output-on-failure
  All 75 tests in that label passed.

Are there any user-facing changes?

No.

Github Issue: 48931

• GitHub Issue: [C++] Optimize Decimal128 abs hotspot; add exec‑only TPC‑H Q1 benchmark #48931

• GitHub Issue: GH-49315: [C++] Optimize Decimal128 abs; add exec-only TPC-H Q1 benchmark #49315

[github-actions]

⚠️ GitHub issue #49315 has been automatically assigned in GitHub to PR creator.

[rok]

Thanks for opening a PR @Manas103 ! Before we start the review, can you share what you're seeing benchmark result improvements after this change?
Also we don't normally reopen PRs for visibility.