Fix pandas_to_eland append to an aliased index

[vismaytiwari]

What

pandas_to_eland(..., es_if_exists="append") raised KeyError when es_dest_index is an alias rather than a concrete index name.

The append path verifies mapping compatibility via:

dest_mapping = es_client.indices.get_mapping(index=es_dest_index)[es_dest_index]

indices.get_mapping keys its response by the concrete index name, which differs from the alias passed in. So for an alias (e.g. an ILM rollover write alias) the subscript [es_dest_index] misses and raises KeyError: '<alias>', exactly as reported in #747.

Fix

Iterate the returned mappings instead of assuming the response is keyed by es_dest_index:

for dest_mapping in es_client.indices.get_mapping(index=es_dest_index).values():
    verify_mapping_compatibility(...)

This keeps the existing behavior for a concrete index (a single entry whose key equals the index), fixes the alias case, and also handles an alias that resolves to more than one index (each resolved index is verified).

Tests

Added test_es_if_exists_append_to_alias in tests/etl/test_pandas_to_eland.py: it creates an index, points an alias at it, and appends through the alias. The test fails on main with KeyError and passes with this change.

Closes #747

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[vismaytiwari]

Gentle nudge - CLA is signed and CI/Snyk are green. Happy to change anything you'd like; otherwise it's ready for a review whenever someone has time.