fix: introduce PreciseTimestamp to fix mysql datetime precision issue.

Copybara import of the project:

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c62a0a6da4317497b991f83d4d8561bdbe7292aa by Jacky W <wjx_colstu@hotmail.com>:

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59c0606ac08ddb45ce469a4d1e1c6e515ef94df4 by Jacky W <wjx_colstu@hotmail.com>:

fix: add cache_ok option to remove sa warning.

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1922599075af935a63544762765d6f0e6224907a by Jacky W <wjx_colstu@hotmail.com>:

chore: format code.
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/1001 from Colstuwjx:fix/issue-480-precise-mysql-timestamp 50e764fb9646e326ad0204ff7adb2a5b65bf875a
PiperOrigin-RevId: 764853016

src/google/adk/sessions/database_session_service.py

@@ -92,6 +92,18 @@ class DynamicJSON(TypeDecorator):
     return value
 
 
+class PreciseTimestamp(TypeDecorator):
+  """Represents a timestamp precise to the microsecond."""
+
+  impl = DateTime
+  cache_ok = True
+
+  def load_dialect_impl(self, dialect):
+    if dialect.name == "mysql":
+      return dialect.type_descriptor(mysql.DATETIME(fsp=6))
+    return self.impl
+
+
 class Base(DeclarativeBase):
   """Base class for database tables."""
 
@@ -156,7 +168,9 @@ class StorageEvent(Base):
   branch: Mapped[str] = mapped_column(
       String(DEFAULT_MAX_VARCHAR_LENGTH), nullable=True
   )
-  timestamp: Mapped[DateTime] = mapped_column(DateTime(), default=func.now())
+  timestamp: Mapped[PreciseTimestamp] = mapped_column(
+      PreciseTimestamp, default=func.now()
+  )
   content: Mapped[dict[str, Any]] = mapped_column(DynamicJSON, nullable=True)
   actions: Mapped[MutableDict[str, Any]] = mapped_column(PickleType)