uv audit: add context/warnings for ignored vulnerabilities

[woodruffw]

Summary

This makes two small QoL changes to uv audit:

• We now warn the user if they ignore (via CLI or config) a vulnerability ID, but that ID doesn't actually match any known vulnerabilities discovered during the audit. This can happen due to drift (e.g. the user upgrades but forgets to removed a stale ID) or user error (the user typos a vulnerability ID).
• We now ignore the number of ignored vulnerabilities as a statistic in the output. In practice, this means users will see something like "5 vulnerabilities (2 ignored)" in the header of uv audit's output if they ignore vulnerabilities.

See #18506.

Test Plan

Added integration tests for the new behavior.