Replace isinstance chains with match/case for observation dispatch

[ecomodeller]

matching.py already uses match/case for observation type dispatch, but several other files still use isinstance chains:

• model/dfsu.py — extract() method
• model/grid.py — extract() method
• model/dummy.py — extract() method
• plotting/_spatial_overview.py — observation type for plotting

These should be converted to match/case for consistency and readability.