[Feature Request]: Optimized GCS Direct Write for Dense Keys #40533

What would you like to happen?

In streaming, WriteFiles shuffles every record before writing temp files:

• fixed sharding: GroupByKey in WriteShardedBundlesToTempFiles
• runner-determined/auto sharding: GroupIntoBatches in WriteAutoShardedBundlesToTempFiles

When input is dense (bundles are large), this shuffle is unnecessary
cost: each bundle is already big enough to produce a reasonably sized
file on its own.

Proposal (similar to the IcebergIO direct-write bypass, #36720):

• Buffer each bundle's records up to a byte threshold.
• Bundles that reach the threshold are written directly to temp files,
  skipping the shuffle.
• Smaller bundles keep the existing grouped path, so files stay large.
• Both paths are merged before finalization, which assigns shard numbers
  across all files (file count per window may differ from the
  runner-chosen shard count).

Enablement (off by default):

• WriteFiles.withDirectWriteByteThreshold(long)
• Pipeline experiment streaming_file_direct_write_threshold_bytes=
  (does not override an explicit user withNumShards(N))
• Follow-up: expose the option on TextIO/FileIO.

Scope:

• Unbounded (streaming) inputs only.
• Disabled when dynamic destinations use side inputs.
• Not GCS-specific: works for any FileSystem, but GCS streaming sinks
  are the main target.

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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