fix(commands): resolve unstringified annotation before caching

[shiftinv]

Summary

utils.evaluate_annotation uses a (per-signature) cache to speed up resolving forwardrefs.
This would previously cache the eval'd result before continuing to resolve it, meaning subsequent calls with the same stringified annotation return the (incomplete) cached result.

from __future__ import annotations

@bot.command()
async def cachetest(ctx: commands.Context, a: Union['int', 'str'], b: Union['int', 'str']) -> None:
    ...

# In both cases, we resolve the outer forwardref ("Union[...]" -> Union[...]),
# but only resolve the inner ones in the first call:
#
# 'ctx': <Parameter "ctx: disnake.ext.commands.context.Context">
# 'a': <Parameter "a: Union[int, str]">
# 'b': <Parameter "b: Union[ForwardRef('int'), ForwardRef('str')]">

The same thing happens with other unique evaluate_annotation behaviors, like turning UnionTypes into Unions.

The correct behavior here is to cache after we've done all the remaining steps.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)