Adding checks to zarr append

[seboceanum]

Added append-coordinate validation in oceanum-python/src/oceanum/datamesh/zarr.py:

• Incoming append coordinate must be monotonic non-decreasing.
• Non-contiguous overlap indices in existing data are rejected.
• Overlap timestamp mismatch is rejected with an explicit “insertion not supported” style message.
• Replaced deprecated drop(...) with drop_vars(..., errors="ignore") in oceanum-python/src/oceanum/datamesh/zarr.py
• Adding deprecation warning when gateway is set to gateway.datamesh.oceanum.io/tech (All v1 logic has already been removed)