add upgrading guide

Author: adriangb

docs/source/library-user-guide/upgrading.md

@@ -182,6 +182,86 @@ let indices = projection_exprs.column_indices();
 _execution plan_ of the query. With this release, `DESCRIBE query` now outputs
 the computed _schema_ of the query, consistent with the behavior of `DESCRIBE table_name`.
 
+### Refactoring of `FileSource` constructors and `FileScanConfigBuilder` to accept schemas upfront
+
+The way schemas are passed to file sources and scan configurations has been significantly refactored. File sources now require the schema (including partition columns) to be provided at construction time, and `FileScanConfigBuilder` no longer takes a separate schema parameter.
+
+**Who is affected:**
+
+- Users who create `FileScanConfig` or file sources (`ParquetSource`, `CsvSource`, `JsonSource`, `AvroSource`) directly
+- Users who implement custom `FileFormat` implementations
+
+**Key changes:**
+
+1. **FileSource constructors now require TableSchema**: All built-in file sources now take the schema in their constructor:
+   ```diff
+   - let source = ParquetSource::default();
+   + let source = ParquetSource::new(table_schema);
+   ```
+
+2. **FileScanConfigBuilder no longer takes schema as a parameter**: The schema is now passed via the FileSource:
+   ```diff
+   - FileScanConfigBuilder::new(url, schema, source)
+   + FileScanConfigBuilder::new(url, source)
+   ```
+
+3. **Partition columns are now part of TableSchema**: The `with_table_partition_cols()` method has been removed from `FileScanConfigBuilder`. Partition columns are now passed as part of the `TableSchema` to the FileSource constructor:
+   ```diff
+   + let table_schema = TableSchema::new(
+   +     file_schema,
+   +     vec![Arc::new(Field::new("date", DataType::Utf8, false))],
+   + );
+   + let source = ParquetSource::new(table_schema);
+     let config = FileScanConfigBuilder::new(url, source)
+   -     .with_table_partition_cols(vec![Field::new("date", DataType::Utf8, false)])
+         .with_file(partitioned_file)
+         .build();
+   ```
+
+4. **FileFormat::file_source() now takes TableSchema parameter**: Custom `FileFormat` implementations must be updated:
+   ```diff
+   impl FileFormat for MyFileFormat {
+   -   fn file_source(&self) -> Arc<dyn FileSource> {
+   +   fn file_source(&self, table_schema: TableSchema) -> Arc<dyn FileSource> {
+   -       Arc::new(MyFileSource::default())
+   +       Arc::new(MyFileSource::new(table_schema))
+       }
+   }
+   ```
+
+**Migration examples:**
+
+For Parquet files:
+```diff
+- let source = Arc::new(ParquetSource::default());
+- let config = FileScanConfigBuilder::new(url, schema, source)
++ let table_schema = TableSchema::new(schema, vec![]);
++ let source = Arc::new(ParquetSource::new(table_schema));
++ let config = FileScanConfigBuilder::new(url, source)
+      .with_file(partitioned_file)
+      .build();
+```
+
+For CSV files with partition columns:
+```diff
+- let source = Arc::new(CsvSource::new(true, b',', b'"'));
+- let config = FileScanConfigBuilder::new(url, file_schema, source)
+-     .with_table_partition_cols(vec![Field::new("year", DataType::Int32, false)])
++ let options = CsvOptions {
++     has_header: Some(true),
++     delimiter: b',',
++     quote: b'"',
++     ..Default::default()
++ };
++ let table_schema = TableSchema::new(
++     file_schema,
++     vec![Arc::new(Field::new("year", DataType::Int32, false))],
++ );
++ let source = Arc::new(CsvSource::new(table_schema).with_csv_options(options));
++ let config = FileScanConfigBuilder::new(url, source)
+      .build();
+```
+
 ### Introduction of `TableSchema` and changes to `FileSource::with_schema()` method
 
 A new `TableSchema` struct has been introduced in the `datafusion-datasource` crate to better manage table schemas with partition columns. This struct helps distinguish between: