fix(grpc): transport validation and error message

[malladinagarjuna2]

fixes #1014
GrpcTransport- getTaskPushNotificationConfiguration(): Replaced a manual IllegalArgumentException null check with checkNotNullParam("id", request.id()) for consistency with the validation used throughout the class.

• subscribeToTask(): Fixed a copy-paste error in the error message prefix — it incorrectly read "Failed to subscribe task push notification config" and now reads "Failed to subscribe to task".

DataPart immutability (regression from #633)

PR #633 widened DataPart.data from Map<String, Object> to Object to support any JSON value (arrays, primitives), but in doing so dropped the data = Map.copyOf(data) defensive copy. As a result, callers could mutate the contents
of a DataPart after construction.

The widening itself is correct — the spec allows any JSON value — so the defensive copy is now applied based on the runtime type:

• Map → CollectionCopies.unmodifiableShallowMap() (preserves null values, consistent with how metadata is already handled)
• List → Collections.unmodifiableList(new ArrayList<>(list)) (preserves null elements)
• Primitives / immutable values → stored as-isAlso corrected the DataPart Javadoc, which claimed data "supports null" while the constructor calls checkNotNullParam and throws on null. The null-value claim is removed from the
  class-level docs and both constructor @param tags.

Tests- Added5 tests in DataPartTest verifying:

• Map/List data is defensively copied and immutable (mutation throws UnsupportedOperationException, later mutation of the original input has no effect)
• Null values in data maps and null elements in data lists survive the copy - Primitives are stored as-is (same instance)
• Full spec module test suite passes (142 tests).

[malladinagarjuna2]

@kabir could you please verify