Test and fix lifetimes of transform_mpi values

[msimberg]

The transform_mpi sender adaptor was not ensuring that the references passed to user-provided MPI callables are kept alive until the MPI request completes. This fixes that by adding an explicit let_value around the helper adaptors of transform_mpi. This PR also adds a number of new tests that trigger the issue if the let_value is not in place.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.06% (target: -1.00%)	✅ 100.00% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (22d5654)	18263	13782	75.46%
Head commit (18b0162)	18272 (+9)	13800 (+18)	75.53% (+0.06%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1321)	9	9	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

See your quality gate settings    Change summary preferences

_{Codacy stopped sending the deprecated coverage status on June 5th, 2024. Learn more}

[biddisco]

Nice. Thanks. much cleaner

[aurianer]

LGTM thanks!

[msimberg]

Increasing the number of test repetitions for sanitizers/valgrind and more aggressively resetting operation states passed tests just fine on 0e6cac7. I'll go ahead and try to merge this.