Fix Arrow decimal type conversion to float instead of integer

[yashwantbezawada]

Summary

• Fixes Arrow Decimal128 columns being incorrectly converted to integers
• Values like 3.14 now display correctly instead of as their internal representation (e.g., 314)
• Maps decimal Arrow types to DTYPE_FLOAT64 and uses Decimal128::ToDouble(scale) for proper conversion

Test plan

• Updated existing decimal stream and legacy tests to expect "float" schema type
• Verify with reproduction code from issue:

  import perspective
  import duckdb
  import pyarrow as pa
  import io
  
  table = duckdb.sql("SELECT 3.14::DECIMAL AS decimal_col, 3.14::FLOAT AS float_col").arrow()
  buffer = io.BytesIO()
  
  with pa.ipc.new_file(buffer, table.schema) as writer:
      writer.write_table(table)
  
  bytes_object = buffer.getvalue()
  
  perspective_table = perspective.Table(bytes_object)
  # Both columns should now show 3.14

Fixes #2582

[yashwantbezawada]

The reproduction code verification is pending CI - the build environment requires CMake 3.29.5+, Emscripten, and various system dependencies for the C++/Rust/WebAssembly compilation.

The fix has been thoroughly code-reviewed:

• convert_type() now maps "decimal"/"decimal128" to DTYPE_FLOAT64 instead of DTYPE_INT64
• copy_array() now uses Decimal128::ToDouble(scale) to properly convert values respecting the decimal scale
• Existing decimal tests have been updated to expect "float" schema type

The CI pipeline should verify the fix works correctly with the full test suite.

[texodus]

Thanks for the PR! Looks good!