fix(params): avoid calling signature() twice

[shiftinv]

Summary

This is a microoptimization for application command signature parsing, passing down the inspect.Signature object through expand_params -> collect_params -> isolate_self, instead of computing a new signature object there.
Also avoids emitting warnings from within string annotations twice, as seen in #991 (comment).

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, ...)