refactor(MeasureTheory): golf Mathlib/MeasureTheory/Function/L2Space

[yuanyi-350]

• rewrites inner_indicatorConstLp_eq_setIntegral_inner to pass through integral_indicator and a single integral_congr_ae argument
• reuses indicatorConstLp_coeFn directly, replacing the separate s and sᶜ integral arguments with a shorter pointwise proof

Extracted from #38104

[github-actions]

PR summary 7705510fbc

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[yuanyi-350]

I ran a profiler comparison for the changed declarations in this PR.

Results (seconds):

• MeasureTheory.L2.inner_indicatorConstLp_eq_setIntegral_inner: 0.375617 -> 0.047875 (-87.25%)

Overall:

• inner_indicatorConstLp_eq_setIntegral_inner is substantially faster after the refactor.

[yuanyi-350]

!bench

[leanprover-radar]

Benchmark results for 159c0f0 against 7705510 are in. No significant results found. @yuanyi-350

• 🟥 build//instructions: +13.9G (+0.01%)

Small changes (1✅)

• ✅ build/module/Mathlib.MeasureTheory.Function.L2Space//instructions: -2.2G (-7.44%)