Closes #5141: Index pandas extension accessor add from_return_msg and other functions

[ajpotts]

Overview

This PR improves the pandas-facing Index integration by extending the Index.ak accessor with thin delegations to legacy Arkouda Index / MultiIndex functionality, while keeping the public surface area aligned with pandas semantics.

Changes

• Add delegation methods on the pandas Index.ak accessor for:
  • concat
  • lookup
  • to_csv, to_parquet, and to_hdf
• Preserve index name / names during concatenation for both Index and MultiIndex.
• Clarify and harden MultiIndex.lookup behavior:
  • Explicit validation of key length
  • Clear separation of scalar-key vs array-key semantics
• Add minimal, focused IO tests that validate round-tripping a pd.Index
  backed by an Arkouda ExtensionArray through:
  • CSV
  • Parquet
  • HDF5

Testing

• New tests live in tests/pandas/extension/io/index_accessor_io.py
• Tests intentionally target only the pandas Index + ExtensionArray surface
  and avoid direct use of legacy ak.Index APIs.

Closes #5141: Index pandas extension accessor add from_return_msg and other functions

[codecov-commenter]

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Additional details and impacted files

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