docs: widen sequence diagram note boxes in auth and push-pull request flows

What does this MR do?

The note boxes in the Mermaid sequence diagrams in docs/auth-request-flow.md and docs/push-pull-request-flow.md were anchored to a single participant (Note right of C / Note left of R), which gives the box a fixed narrow width that long text overflows.

This changes them to span two participants (Note over ...) so Mermaid sizes the box to the span and wraps the text inside it, preventing overflow.

Documentation-only change.

Author checklist

• Assign one of conventional-commit prefixes to the MR.
  • docs: Updates or changes to documentation. Does not trigger a release.