feat: Database forward compatibility: make owner field optional (#812)

# Description
Old version is not able to write in databases which are `1.0` compatible
because the new `owner` field is mandatory. This change makes the
`owner` field `optional` to maintain forward compatibility.

## Tested 
### Before change (owner field mandatory)
I created a database with tables `tasks` and `push_notification_configs`
using `1.0` spec. I then tested it against `0.3` spec by trying to
**write** in it. It failed returning an error: `NOT NULL constraint
failed: tasks.owner`.

Also, I created a database with `0.3` spec `Task` and
`PushNotificationConfigs` entries but then made it `1.0` compatible via
migration CLI command `uv run a2a-db`. **Reading** such data using `0.3`
spec worked, but **writing** failed for the same reason as before.
### After change (owner field optional)
I repeated the testing from before. Now writing in `1.0` compatible
databases worked.

## Notes
Writing in database which was created using `1.0` spec without running
the migration `a2a-db` CLI command, created entries which have
`owner=NULL` values, contrary to databases that underwent the migration
using `a2a-db` which have `owner` field populated with the default
value. In my case with `legacy_v03_no_user_info` since I didn't use the
`--add_columns_owner_last_updated-default-owner` flag to pass my own
custom default value.


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Fixes #811 🦕

Author: sokoliva

src/a2a/a2a_db_cli.py

@@ -58,7 +58,7 @@ def create_parser() -> argparse.ArgumentParser:
     parser.add_argument(
         '--add_columns_owner_last_updated-default-owner',
         dest='owner',
-        help="Value for the 'owner' column (used in specific migrations). If not set defaults to 'unknown'",
+        help="Value for the 'owner' column (used in specific migrations). If not set defaults to 'legacy_v03_no_user_info'",
     )
     _add_shared_args(parser)
 

src/a2a/migrations/versions/6419d2d130f6_add_columns_owner_last_updated.py

@@ -47,7 +47,7 @@ def upgrade() -> None:
     )
 
     if table_exists(tasks_table):
-        add_column(tasks_table, 'owner', False, sa.String(255), owner)
+        add_column(tasks_table, 'owner', True, sa.String(255), owner)
         add_column(tasks_table, 'last_updated', True, sa.DateTime())
         add_index(
             tasks_table,
@@ -63,7 +63,7 @@ def upgrade() -> None:
         add_column(
             push_notification_configs_table,
             'owner',
-            False,
+            True,
             sa.String(255),
             owner,
         )

src/a2a/server/models.py

@@ -44,7 +44,7 @@ class TaskMixin:
     kind: Mapped[str] = mapped_column(
         String(16), nullable=False, default='task'
     )
-    owner: Mapped[str] = mapped_column(String(255), nullable=False)
+    owner: Mapped[str] = mapped_column(String(255), nullable=True)
     last_updated: Mapped[datetime | None] = mapped_column(
         DateTime, nullable=True
     )
@@ -145,7 +145,7 @@ class PushNotificationConfigMixin:
     task_id: Mapped[str] = mapped_column(String(36), primary_key=True)
     config_id: Mapped[str] = mapped_column(String(255), primary_key=True)
     config_data: Mapped[bytes] = mapped_column(LargeBinary, nullable=False)
-    owner: Mapped[str] = mapped_column(String(255), nullable=False, index=True)
+    owner: Mapped[str] = mapped_column(String(255), nullable=True, index=True)
     protocol_version: Mapped[str | None] = mapped_column(
         String(16), nullable=True
     )