[FEATURE] Auto-adjust diagram zoom/pan to the participants involved in the current step

# Feature

In the OAuth Debugger's sequence diagram, when the interaction moves between participants that are far apart (e.g. Step 5 switches from Client–MCP Server to Client–Authorization Server), the diagram view doesn't follow. Since the two participants sit far apart horizontally, the current pan/zoom only shows the empty space between them and misses the actual message content, as shown in the attached screenshot.

# Solution

Automatically adjust zoom/pan (fit-to-view or center-and-zoom) to the two participants relevant to the currently active step whenever the step changes, so the interaction is always fully visible without manual scrolling/zooming.

# Alternatives

- Keep manual zoom, but add a "Focus this step" action (distinct from the existing "Show in diagram") that recenters the view.
- Add a mini-map indicator showing which part of the diagram is off-screen.

# Additional Context

Screenshot attached showing Step 5's interaction only partially visible in the diagram.

Start in the OAuth Debugger's sequence diagram and trace how the currently active step changes the displayed interaction. Read the existing "Show in diagram" behavior before deciding where view changes belong. Done means switching between steps keeps both relevant participants and their message content visible without manual panning or zooming.

Written by the indexing model from the issue text.