Propagate schema.name-mapping.default from table metadata into FileScanTask

[prakharjain09]

Is your feature request related to a problem or challenge?

FileScanTask.name_mapping is always set to None during scan planning,
even when the table sets the schema.name-mapping.default property. This
leaves a TODO in the code:

https://github.com/apache/iceberg-rust/blob/main/crates/iceberg/src/scan/cont
ext.rs#L140

// TODO: Extract name_mapping from table metadata property
"schema.name-mapping.default"
name_mapping: None,

Impact

Readers rely on the name mapping to resolve field IDs for Parquet files that
either lack field IDs or have conflicting ones — the existing
record-batch-transformer and projection code already handles
Some(NameMapping) correctly, but the value never reaches it from the planner.
 As a result, **scans of tables that depend on schema.name-mapping.default
(e.g. tables migrated from Hive, files written without Iceberg field-ID
metadata) silently fall back to position-based ID assignment**.

### Describe the solution you'd like

Parse the schema.name-mapping.default property once in
TableScanBuilder::build, store the resulting Arc<NameMapping> on PlanContext,
 and thread it through ManifestFileContext and ManifestEntryContext so it
lands on FileScanTask. Malformed JSON should surface as
ErrorKind::DataInvalid from build() rather than being silently dropped.

### Willingness to contribute

I would be willing to contribute to this feature with guidance from the Iceberg Rust community