chore: expose symbols (#7451)

* chore/expose-symbols:
 ### Commit Message

 Enhance `merge_and_dedup` Functionality in `flush.rs`

 - **Function Signature Update**: Modified the `merge_and_dedup` function to accept `append_mode` and `merge_mode` as separate parameters instead of using `options`.
 - **Function Accessibility**: Changed the visibility of `merge_and_dedup` to `pub` to allow external access.
 - **Function Calls Update**: Updated calls to `merge_and_dedup` within `memtable_flat_sources` to align with the new function signature, passing `options.append_mode` and `options.merge_mode()` directly.

Signed-off-by: Lei, HUANG <mrsatangel@gmail.com>

* chore/expose-symbols:
 ### Add Merge and Deduplication Functionality

 - **File**: `src/mito2/src/flush.rs`
   - Introduced `merge_and_dedup` function to merge multiple record batch iterators and apply deduplication based on specified modes.
   - Added detailed documentation for the function, explaining its arguments, behavior, and usage examples.

Signed-off-by: Lei, HUANG <mrsatangel@gmail.com>

---------

Signed-off-by: Lei, HUANG <mrsatangel@gmail.com>

src/mito2/src/flush.rs

@@ -801,7 +801,8 @@ fn memtable_flat_sources(
             if last_iter_rows > min_flush_rows {
                 let maybe_dedup = merge_and_dedup(
                     &schema,
-                    options,
+                    options.append_mode,
+                    options.merge_mode(),
                     field_column_start,
                     std::mem::replace(&mut input_iters, Vec::with_capacity(num_ranges)),
                 )?;
@@ -813,7 +814,13 @@ fn memtable_flat_sources(
 
         // Handle remaining iters.
         if !input_iters.is_empty() {
-            let maybe_dedup = merge_and_dedup(&schema, options, field_column_start, input_iters)?;
+            let maybe_dedup = merge_and_dedup(
+                &schema,
+                options.append_mode,
+                options.merge_mode(),
+                field_column_start,
+                input_iters,
+            )?;
 
             flat_sources.sources.push(FlatSource::Iter(maybe_dedup));
         }
@@ -822,19 +829,64 @@ fn memtable_flat_sources(
     Ok(flat_sources)
 }
 
-fn merge_and_dedup(
+/// Merges multiple record batch iterators and applies deduplication based on the specified mode.
+///
+/// This function is used during the flush process to combine data from multiple memtable ranges
+/// into a single stream while handling duplicate records according to the configured merge strategy.
+///
+/// # Arguments
+///
+/// * `schema` - The Arrow schema reference that defines the structure of the record batches
+/// * `append_mode` - When true, no deduplication is performed and all records are preserved.
+///                  This is used for append-only workloads where duplicate handling is not required.
+/// * `merge_mode` - The strategy used for deduplication when not in append mode:
+///   - `MergeMode::LastRow`: Keeps the last record for each primary key
+///   - `MergeMode::LastNonNull`: Keeps the last non-null values for each field
+/// * `field_column_start` - The starting column index for fields in the record batch.
+///                          Used when `MergeMode::LastNonNull` to identify which columns
+///                          contain field values versus primary key columns.
+/// * `input_iters` - A vector of record batch iterators to be merged and deduplicated
+///
+/// # Returns
+///
+/// Returns a boxed record batch iterator that yields the merged and potentially deduplicated
+/// record batches.
+///
+/// # Behavior
+///
+/// 1. Creates a `FlatMergeIterator` to merge all input iterators in sorted order based on
+///    primary key and timestamp
+/// 2. If `append_mode` is true, returns the merge iterator directly without deduplication
+/// 3. If `append_mode` is false, wraps the merge iterator with a `FlatDedupIterator` that
+///    applies the specified merge mode:
+///    - `LastRow`: Removes duplicate rows, keeping only the last one
+///    - `LastNonNull`: Removes duplicates but preserves the last non-null value for each field
+///
+/// # Examples
+///
+/// ```ignore
+/// let merged_iter = merge_and_dedup(
+///     &schema,
+///     false,  // not append mode, apply dedup
+///     MergeMode::LastRow,
+///     2,  // fields start at column 2 after primary key columns
+///     vec![iter1, iter2, iter3],
+/// )?;
+/// ```
+pub fn merge_and_dedup(
     schema: &SchemaRef,
-    options: &RegionOptions,
+    append_mode: bool,
+    merge_mode: MergeMode,
     field_column_start: usize,
     input_iters: Vec<BoxedRecordBatchIterator>,
 ) -> Result<BoxedRecordBatchIterator> {
     let merge_iter = FlatMergeIterator::new(schema.clone(), input_iters, DEFAULT_READ_BATCH_SIZE)?;
-    let maybe_dedup = if options.append_mode {
+    let maybe_dedup = if append_mode {
         // No dedup in append mode
         Box::new(merge_iter) as _
     } else {
         // Dedup according to merge mode.
-        match options.merge_mode() {
+        match merge_mode {
             MergeMode::LastRow => {
                 Box::new(FlatDedupIterator::new(merge_iter, FlatLastRow::new(false))) as _
             }