feat!: Merge createDataset and addPieces for streamlined dataset creation

[Chaitu-Tatipamula]

Description

This PR implements the feature described in #171 at filecoin-services to merge createDataset and addPieces operations for a streamlined user experience.

Changes

PDP Verifier Updates

• Enhanced addPieces function: Now accepts (datasetID, listenerAddress, pieces, extraData) parameters
• New dataset creation: Use datasetID = NEW_DATA_SET_SENTINEL (type(uint256).max) with a valid listenerAddress to create a new dataset and add pieces in a single transaction
• Existing dataset updates: Use listenerAddress = 0 with an existing datasetID to add pieces to existing datasets
• Removed createDataset method: No longer needed as functionality is merged into addPieces

Test Improvements

• Added comprehensive test coverage for the combined create-and-add flow
• Fixed listener initialization issues across test contracts
• Added TestConstants.sol for shared constants like NEW_DATA_SET_SENTINEL
• Updated all test contracts to use the new streamlined approach

Benefits

• ✅ Single transaction: Users can create datasets and add pieces in one operation
• ✅ Reduced wait times: No need to wait for dataset creation confirmation before adding pieces
• ✅ Simplified UX: One client signing operation instead of two
• ✅ Backward compatibility: Existing functionality for adding to existing datasets preserved

Testing

• All existing tests pass
• New tests cover the combined create-and-add flow
• Listener initialization issues resolved
• Event emission verified for both dataset creation and piece addition

Related Issue

Closes #171 at filecoin-services

[wjmelements]

use nPieces here too, and also in the line below.