Replace `DataProxy.get_metadata` with `get_length` and `get_aliases`

[theferrit32]

https://github.com/ga4gh/vrs-python/blob/2.3.1/src/ga4gh/vrs/dataproxy.py#L53-L70

The only fields in the return used by vrs-python are length and aliases.

The vrs docs (https://github.com/ga4gh/vrs/blob/2.1.0-snapshot.2026-02.2/docs/source/conventions/required_data.rst#suggested-implementation) say

get_metadata(identifier): Given a sequence identifier, return a dictionary of length, alphabet, and known aliases.

But also

To get sequence length, aliases, and other optional information for a given identifier:

We should really be explicit about what is actually needed by vrs-python, so that the interface contract is explicit and other data providers can implement them and be sure it fulfills what is needed.

If length and aliases are all that is needed I don't think we need a "metadata" method. alphabet is not used anywhere in vrs-python. If we do not have plans to use it, we should drop it from the docs and just go with get_length and get_aliases methods, not an unneeded wrapper which composes the return values of those into a dict.

[jsstevenson]

Note that this would represent further departure from the seqrepo dataproxy API

[theferrit32]

I think that's okay. Generally think we should try to move away from "suggestions" and dicts which seem like they may have (or should have) certain fields, but aren't required to have them and which the codebase in question makes no reference to.

[theferrit32]

If we proceed with this change we should also update

https://github.com/ga4gh/vrs/blob/2.1.0-snapshot.2026-02.2/docs/source/conventions/required_data.rst#suggested-implementation

[jsstevenson]

https://github.com/ga4gh/vrs/blob/2.1.0-snapshot.2026-02.2/docs/source/conventions/required_data.rst#suggested-implementation

Yeah I don't think that section needs to be there at all. VRS-Clojure can figure out how it wants to do things by itself.