fix: use MySQL compatible syntax in list tasks (#651)

# Description

Do not use `NULLS LAST` which is not available in MySQL, coalesce nulls
to empty strings which will appear last in descending ordering (there
are tests for this behavior already, however CI wasn't enabled for this
branch).

Currently `NULLS LAST` fails MySQL tests:

```
(1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NULLS LAST, tasks.id DESC \n LIMIT 51' at line 3")
```
([actions
run](https://github.com/a2aproject/a2a-python/actions/runs/21520017047/job/62008309612?pr=651))

Enable tests run against `1.0-dev` to prevent it in the future.

Re #511 
Fixes #652 

---
Mark as "refactor" for release please as it's a fix for a non-released
feature, hence shouldn't get into a changelog.

BEGIN_COMMIT_OVERRIDE
refactor: use MySQL compatible syntax in list tasks
END_COMMIT_OVERRIDE

Author: ishymko

.github/workflows/unit-tests.yml

@@ -2,7 +2,7 @@
 name: Run Unit Tests
 on:
   pull_request:
-    branches: [main]
+    branches: [main, 1.0-dev]
 permissions:
   contents: read
 jobs:

src/a2a/server/tasks/database_task_store.py

@@ -189,8 +189,10 @@ async def list(
             count_stmt = select(func.count()).select_from(base_stmt.alias())
             total_count = (await session.execute(count_stmt)).scalar_one()
 
+            # Use coalesce to treat NULL timestamps as empty strings,
+            # which sort last in descending order
             stmt = base_stmt.order_by(
-                timestamp_col.desc().nulls_last(),
+                func.coalesce(timestamp_col, '').desc(),
                 self.task_model.id.desc(),
             )