feat(migrations): add revision for task version column and task_events table

[mykytanetipa]

Adds the Alembic revision that creates the schema the cluster stores need. #1244 adds it to the models, which affects deployments that rely on SDKs DB task store implementation;

Key changes:

• Revision b5e3d1c8a2f7 (revises 38ce57e08137).
• Adds the version column to the tasks table and gives existing rows a starting value, so tasks already in flight at upgrade time can still be updated afterwards.
• Creates the task_events table with an index on task_id.

Tested:

• Unit tests for the full up/down cycle, the backfill, a missing tasks table and offline SQL generation.
• test: add multi-server scenarios over a shared database #1247 adds integration tests that exercise the resulting schema against a real database.

Design discussion: #1224

[github-actions]

🧪 Code Coverage (vs multiserver/sample)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/agent_execution/active_task.py	94.18%	94.63%	🟢 +0.45%
src/a2a/migrations/versions/b5e3d1c8a2f7_add_task_version_and_task_events.py (new)	—	75.00%	—
Total	92.85%	92.81%	🔴 -0.05%

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