Frameshift variants should not return valid VRS objects in gnomad_vcf_to_protein

### Describe the bug

VRS does not currently support variants where there is a resulting frameshift. We should detect this and return an error. Currently, we return incorrect VRS objects in cases of frameshifts

### Steps to reproduce

1. Attempt to normalize vcf coords that are known to result in a frameshift in the /gnomad_vcf_to_protein endpoint (ex: 17-43124028-CTCT-CT)
2. notice that there is a VRS object returned

### Expected behavior

We should not return a VRS Allele since VRS does not support fs variants currently. We should return a warning saying "Frameshift variants are currently not supported" or something of that sort

### Current behavior

valid vrs allele returned

### Possible reason(s)

we don't check for it

### Suggested fix

check for if a variant results in a frameshift before converting/returning

### Branch, commit, and/or version

any

### Screenshots

_No response_

### Environment details

Not applicable

### Additional details

:)

### Contribution

None

Start by reproducing the /gnomad_vcf_to_protein request with 17-43124028-CTCT-CT and trace the endpoint's variant conversion path. Confirm that frameshift results produce a warning such as "Frameshift variants are currently not supported" and no VRS Allele, then run the relevant endpoint tests if available.

Written by the indexing model from the issue text.