refactor(MeasureTheory): golf Mathlib/MeasureTheory/Covering/Besicovitch

[yuanyi-350]

• shortens the proof of r_t0 in exists_closedBall_covering_tsum_measure_le by turning the contradiction argument into a direct lambda
• replaces the manual simp only proof with mem_iUnion₂ and a final simp [r, this]

Extracted from #38104

[github-actions]

PR summary 2fbf21f9b5

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

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

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

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

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository 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):

• exists_closedBall_covering_tsum_measure_le: 1.530226 -> 1.608270 (+5.10%)

Overall:

• All changed declarations are slower.