fix(doris): support IVF on disk index properties

[zhiqiang-hhhh]

Summary

• Allow Doris ANN index properties to be passed through via --index-prop, including future Doris index types and index-specific options.
• Fix ivf_on_disk index creation by ensuring nlist is preserved and forwarded through Doris SDK ANN properties.
• Update Doris CLI docs to describe generic index property passthrough and remove HNSW-specific convenience CLI options.

Motivation

Doris supports more ANN index types than HNSW, and new index properties may be added in Doris without requiring VectorDBBench changes. Previously the Doris client validated index types locally and only handled HNSW/IVF paths, which made ivf_on_disk and future index properties fragile.
This PR makes Doris index configuration more forward-compatible by treating unknown index types and unmapped index options as passthrough properties.

Changes

• Normalize Doris index_type and default it to hnsw when omitted.
• Merge index_properties before applying known convenience fields.
• Require nlist for known IVF index types: ivf and ivf_on_disk.
• Pass unknown Doris index types through instead of rejecting them locally.
• Attach unmapped index parameters to IndexOptions.ann_properties or IndexOptions.properties.
• Preserve runtime index options from both ann_properties and properties.
• Remove Doris CLI inheritance from HNSW-specific options.
• Document --index-prop index_type=... usage with an ivf_on_disk example.

Testing

• Ran python -m compileall -q vectordb_bench/backend/clients/doris
• Ran git diff --check

[sre-ci-robot]

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