refactor: Remove redundant parameters from SnapshotProducer validation methods (#1853)

## Which issue does this PR close?


- Closes #.

## What changes are included in this PR?



## Summary
Refactor `SnapshotProducer` validation methods to use internal state
instead of requiring redundant parameters.

## Problem
I've noticed that while the current **SnapshotProducer** API design
already equips SnapshotProducer with all necessary state, the current
invocations still redundantly pass parameters externally. I believe this
could lead to some issues.

1. **Data mismatch risk**: Callers could pass different data than what's
stored in `SnapshotProducer`, leading to validating one set of files but
committing another
2. **API complexity**: As more validations are added (e.g., delete
files, file existence checks), each method would require additional
parameters, making the API harder to use
3. **Redundant passing**: The same data that was already provided during
construction has to be passed again

## Changes
- Modified `validate_added_data_files()` and
`validate_duplicate_files()` to operate on `self.added_data_files`
directly
- Updated `FastAppendAction::commit()` to call validation methods
without passing `added_data_files` parameter

## Motivation
Previously, `added_data_files` was passed as a parameter to validation
methods even though it was already stored in `SnapshotProducer`:

```rust
// Before
snapshot_producer.validate_added_data_files(&self.added_data_files)?;

// After  
snapshot_producer.validate_added_data_files()?;
```

## Benefits
1. Better encapsulation - validation operates on object's own state
2. Safer API - eliminates possibility of data mismatch
3. Simpler interface - no redundant parameters needed

## Discussion

Since **SnapshotProducer** already holds all necessary state, can we
further refine validation by performing it during the **new** function's
execution to improve data consistency and encapsulation?


## Are these changes tested?

Author: Li0k

crates/iceberg/src/transaction/append.rs

@@ -93,13 +93,11 @@ impl TransactionAction for FastAppendAction {
         );
 
         // validate added files
-        snapshot_producer.validate_added_data_files(&self.added_data_files)?;
+        snapshot_producer.validate_added_data_files()?;
 
         // Checks duplicate files
         if self.check_duplicate {
-            snapshot_producer
-                .validate_duplicate_files(&self.added_data_files)
-                .await?;
+            snapshot_producer.validate_duplicate_files().await?;
         }
 
         snapshot_producer

crates/iceberg/src/transaction/snapshot.rs

@@ -99,8 +99,8 @@ impl<'a> SnapshotProducer<'a> {
         }
     }
 
-    pub(crate) fn validate_added_data_files(&self, added_data_files: &[DataFile]) -> Result<()> {
-        for data_file in added_data_files {
+    pub(crate) fn validate_added_data_files(&self) -> Result<()> {
+        for data_file in &self.added_data_files {
             if data_file.content_type() != crate::spec::DataContentType::Data {
                 return Err(Error::new(
                     ErrorKind::DataInvalid,
@@ -123,11 +123,9 @@ impl<'a> SnapshotProducer<'a> {
         Ok(())
     }
 
-    pub(crate) async fn validate_duplicate_files(
-        &self,
-        added_data_files: &[DataFile],
-    ) -> Result<()> {
-        let new_files: HashSet<&str> = added_data_files
+    pub(crate) async fn validate_duplicate_files(&self) -> Result<()> {
+        let new_files: HashSet<&str> = self
+            .added_data_files
             .iter()
             .map(|df| df.file_path.as_str())
             .collect();