Fix memory leak in Suzuki-Trotter synthesis (backport #16687)

[mergify]

This Pull request contains changes that will solve the #16677 issue .

The .leak() in file crates/circuit_library/src/suzuki_trotter.rs was only needed because sparse_term_evolution tied the returned iterator's lifetime to the input string. That tie is unnecessary as the function immediately copies everything it needs out of pauli (pauli.to_lowercase() and the derived owned paulis/indices) and never borrows from it afterwards. So I removed the lifetime bound from the crates/synthesis/src/pauli_evolution.rs file

Testing suzuki_trotter file :
cargo +nightly miri test -p qiskit-circuit-library suzuki_trotter::test::test_suzuki_trotter -- --nocapture

running 1 test
test suzuki_trotter::test::test_suzuki_trotter ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 1.50s

Other Testing results :
cargo test -p qiskit-synthesis — passed
cargo test -p qiskit-circuit-library — passed
cargo +nightly miri test -p qiskit-synthesis — no UB/memory issues
cargo +nightly miri test -p qiskit-circuit-library — no UB/memory issues

Fixes #16677

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code:

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This is an automatic backport of pull request #16687 done by [Mergify](https://mergify.com).

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[coveralls]

Coverage Report for CI Build 31508354574

Coverage increased (+0.07%) to 87.741%

Details

• Coverage increased (+0.07%) from the base build.
• Patch coverage: 17 of 17 lines across 2 files are fully covered (100%).
• 3 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

3 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/lex.rs	3	92.54%

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Coverage Stats

Relevant Lines:	128766
Covered Lines:	112981
Line Coverage:	87.74%
Coverage Strength:	997715.06 hits per line

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💛 - Coveralls