feat: add offline mode to rattler CLI

[baszalmstra]

Summary

• Add an offline middleware in rattler_networking that rejects outgoing requests before they reach the rest of the client stack.
• Add a global --offline flag to the rattler CLI and pass it explicitly into commands that create HTTP clients.
• Use cached repodata only for solve/create/search/exec while offline, and avoid auth/upload network calls.

Validation

• cargo fmt --all
• cargo check -p rattler_networking -p rattler-bin
• cargo test -p rattler_networking offline_middleware --lib
• git diff --check

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: pi coding agent

[pavelzw]

Cc @delsner we can use this in pixi-pack