Fix batch send_key crash in #size (use user_key_as_value)

[zuchmanski]

Summary

BatchWrite#size, BatchUDF#size, and BatchDelete#size raised NoMethodError: undefined method 'estimate_size' for an instance of String whenever the batch record's policy had send_key: true. Since #size runs while estimating the wire buffer during batch_operate, any batch with send_key crashed before reaching the network.

Root cause

The three #size methods called @key.user_key.estimate_size, but Key#user_key returns the unwrapped raw value (@user_key.get → String/Integer), and #estimate_size is defined only on Aerospike::Value. The single-record write path in command.rb already does this correctly via user_key_as_value.

Fix

Replace @key.user_key.estimate_size with @key.user_key_as_value.estimate_size in:

• lib/aerospike/batch_write.rb
• lib/aerospike/batch_udf.rb
• lib/aerospike/batch_delete.rb

Verification

Reproduction from the report now runs cleanly for all three classes (previously raised):

BatchWrite:  36    # was: crash
BatchDelete: 17    # was: crash
BatchUDF:    38    # was: crash
control:     25    # send_key omitted, unchanged

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