fix: support explicit db_name in snapshot APIs for cross-database operations

[zhuwenxing]

Summary

• Add explicit db_name parameter to 4 snapshot APIs (create_snapshot, list_snapshots, restore_snapshot, list_restore_snapshot_jobs) across all layers (Prepare, GrpcHandler, AsyncGrpcHandler, MilvusClient, AsyncMilvusClient)
• The db_name is now populated in the protobuf request body, enabling cross-database snapshot operations (e.g., restore a snapshot from db_a into db_b)
• This complements fix: pass database context in all snapshot APIs #3292 which added gRPC metadata context — now db_name is sent via both proto body and metadata header

Why both proto body and metadata?

Path	When it works
Proto body db_name	When user passes explicit db_name parameter (cross-db operations)
Metadata header dbname (via CallContext)	When user omits db_name, falls back to client's self._db_name via DatabaseInterceptor

Affected APIs

API	db_name in proto body
create_snapshot	added
list_snapshots	added
restore_snapshot	added
list_restore_snapshot_jobs	added

Example: cross-database restore

client_a = MilvusClient(uri="...", db_name="db_a")
# Create snapshot in db_a
client_a.create_snapshot(collection_name="my_col", snapshot_name="snap1")

# Restore to a different database
client_a.restore_snapshot(
    snapshot_name="snap1",
    collection_name="restored_col",
    db_name="db_b",  # explicit target database
)

Test plan

• Unit tests: 29 snapshot tests pass (including 4 new cross-db tests)
• Integration tests: 36 scenarios pass against Milvus master (338bac3), covering:
  • 3 databases with different collections/snapshots
  • create_snapshot with explicit db_name from default-db client
  • list_snapshots per database with db_name filter
  • Cross-db restore: db_a → db_b, db_a → db_c (empty db)
  • Same-db restore: db_b → db_b
  • Same-db restore without explicit db_name (uses client's db via metadata)
  • Data isolation verification across all 3 databases

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.79%. Comparing base (5147d51) to head (d226f92).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3295   +/-   ##
=======================================
  Coverage   76.79%   76.79%           
=======================================
  Files          63       63           
  Lines       13236    13236           
=======================================
  Hits        10164    10164           
  Misses       3072     3072           

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[zhuwenxing]

/assign @weiliu1031
/assign @tedxu

[XuanYang-cn]

I think two db_names might not work in Milvus, the API in pymilvus is explicitly designed to omit db_name to reduce the complexity, that all(at least majority) API doesn't need db_name.

[XuanYang-cn]

This is a good catch though, APIs are not aligned with each other on how to pass in db_name. For now, we tends to set the db_name in interceptors only.

[XuanYang-cn]

/hold

[XuanYang-cn]

snapshot will be bind with collection_name and the API will be redesigned soon. Let's make db_name in PyMIlvus consists with other APIs.