Consolidate duplicate ShardedKey implementations in core module

[Eliaaazzz]

Consolidate the two duplicate ShardedKey classes in the Java SDK core module into a single unified implementation. The deprecated org.apache.beam.sdk.values.ShardedKey (already @Deprecated since its introduction) and its internal companion org.apache.beam.sdk.coders.ShardedKeyCoder (@VisibleForTesting) are removed in favor of the single org.apache.beam.sdk.util.ShardedKey.

What changed:

• Removed org.apache.beam.sdk.values.ShardedKey (deprecated, int-based shard) and org.apache.beam.sdk.coders.ShardedKeyCoder (internal, @VisibleForTesting)
• Added of(K, int) factory method and getShardNumber() accessor to org.apache.beam.sdk.util.ShardedKey — the int is stored as a 4-byte big-endian byte array and survives coder round-trips
• Improved toString() to display integer shard number (not raw byte array) for 4-byte shard IDs, preserving log readability
• Migrated all consumers across core SDK (WriteFiles, ShardingFunction), BigQuery IO (BatchLoads, WriteTables, WritePartition, WriteBundlesToFiles, StreamingWriteTables, GenerateShardedTable), Flink runner, and Spark runner
• Simplified auto-sharded write paths in WriteFiles and BatchLoads that previously had awkward conversion code between the two ShardedKey types
• Added tests for int-based shard methods, equality, and coder round-trip
• Updated CHANGES.md with Breaking Changes entry

Wire format note: The removed ShardedKeyCoder had a different wire format from ShardedKey.Coder (reversed field order, VarInt vs ByteArray shard encoding). ShardedKeyCoder was NOT a Beam standard/model coder — it was @VisibleForTesting and only used transiently within pipeline constructions (GroupByKey, sharding), never for persistent state or checkpoints. The standard model coder (ShardedKey.Coder in sdk.util, registered in ModelCoderRegistrar) is unchanged.

Fixes #37990

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request streamlines the Apache Beam Java SDK by consolidating duplicate ShardedKey implementations. By unifying these classes, the change reduces technical debt, eliminates potential confusion regarding shard ID types, and simplifies the auto-sharded write paths in various IO components. The refactoring ensures that existing consumers can continue to use integer-based shard identifiers seamlessly through new helper methods, while improving the overall maintainability of the codebase.

Highlights

• Consolidation of ShardedKey: Merged the two duplicate ShardedKey implementations into a single, unified class located in the core utility package.
• API Improvements: Added a new factory method and accessor for integer-based shard numbers to ensure backward compatibility and ease of use.
• Refactoring and Cleanup: Removed the deprecated ShardedKey and ShardedKeyCoder classes and updated all internal references across the SDK, BigQuery IO, and runners.

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