refactor(mito2): remove PartitionTreeMemtable

[evenyag]

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What's changed and what's your intention?

This PR removes PartitionTreeMemtable.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[gemini-code-assist]

Code Review

This pull request removes the experimental PartitionTreeMemtable implementation and transitions its functionality to BulkMemtable, which now serves as the default for regions using sparse primary key encoding. The changes involve a comprehensive cleanup of the partition_tree module, associated configuration options, metrics, and benchmarks. Feedback highlights that the current implementation for sparse encoding fails to respect user-provided BulkMemtableConfig, overriding it with defaults; it is suggested to update the logic to preserve these configurations when present.

[gemini-code-assist]

When primary_key_encoding is Sparse, the code forces the use of BulkMemtable. However, the current implementation ignores any user-provided BulkMemtableConfig in options.memtable because it initializes the BulkMemtableBuilder with default settings. This means that if a user explicitly configured bulk memtable options (like merge_threshold), those settings will be lost when sparse encoding is active. The logic should be updated to preserve the configuration if options.memtable is already set to Bulk.

Suggested change

	if primary_key_encoding == PrimaryKeyEncoding::Sparse {
	if options.memtable.is_some() {
	common_telemetry::info!(
	"Overriding memtable config, use BulkMemtable for sparse primary key encoding"
	);
	}
	return Arc::new(
	BulkMemtableBuilder::new(self.write_buffer_manager.clone(), !dedup, merge_mode)
	.with_compact_dispatcher(self.compact_dispatcher.clone()),
	);
	}
	if primary_key_encoding == PrimaryKeyEncoding::Sparse {
	let mut builder = BulkMemtableBuilder::new(self.write_buffer_manager.clone(), !dedup, merge_mode)
	.with_compact_dispatcher(self.compact_dispatcher.clone());
	if let Some(MemtableOptions::Bulk(config)) = &options.memtable {
	builder = builder.with_config(config.clone());
	} else if options.memtable.is_some() {
	common_telemetry::info!(
	"Overriding memtable config, use BulkMemtable for sparse primary key encoding"
	);
	}
	return Arc::new(builder);
	}