Write delete manifests with content='deletes' file metadata

[raghav-reglobe]

Problem

WriteToFile in src/core/metadata/manifest/iceberg_manifest.cpp hardcodes the Avro
file-metadata content key to "data" for every manifest it writes, including delete
manifests:

metadata_values.emplace_back("content", "data");

Per the Iceberg spec, a manifest's content metadata must reflect what it holds — "data"
for a data manifest, "deletes" for a delete manifest. So a v3 DELETE (which writes a
deletion-vector delete manifest) produces an Avro file whose content metadata says
data, contradicting the manifest list entry, which correctly records it as a delete
manifest.

Readers that take a manifest's content from the manifest list entry tolerate this (so it
does not affect query results in duckdb-iceberg itself), but spec-strict readers cross-check
the manifest file's own content metadata against the list entry and reject the mismatch.
For example, iceberg-go's ReadManifest errors with:

manifest file's 'content' metadata indicates "data", but entry from manifest list indicates "deletes"

Fix

Derive the value from the manifest's content type (the IcebergManifestFile the function
already receives):

metadata_values.emplace_back("content",
                             manifest_file.content == IcebergManifestContentType::DATA ? "data" : "deletes");

Testing

test/sql/local/catalog_test_config_setup/catalog_agnostic/delete/test_delete_manifest_content_type.test:
after a v3 DELETE, resolves the delete manifest via iceberg_metadata and asserts its
Avro file metadata carries content='deletes' (and not the mislabeled content='data').

Found alongside #1095 — both are v3 deletion-vector write-correctness fixes.

[Tishj]

Thanks 👍