Update alg_props allocation to use `ensure_...` function

alg_props structures are commonly initialized as
```
struct aws_cryptosdk_alg_properties alg_props;

ensure_alg_properties_attempt_allocation(&alg_props);
```
but we have updated `ensure_alg_properties_attemp_allocation` in #638 . Thus we should change the previous code to
```
struct aws_cryptosdk_alg_properties alg_props = ensure_alg_properties_attempt_allocation(MAX_STRING_LEN);
```
in all proofs using alg_props structures.

Search the repository for alg_props structures and ensure_alg_properties_attempt_allocation calls, focusing on the proof code mentioned in the issue. Update each matching initialization to use the MAX_STRING_LEN argument, then run the repository's proof or test checks to confirm all affected code still builds and passes.

Written by the indexing model from the issue text.