Add `--drop_vector_index` to `weaviate-cli update collection`

[jfrancoa]

Context

Weaviate 1.39 adds an experimental endpoint to drop the index of a named vector
(DELETE /v1/schema/{Class}/vectors/{name}/index), exposed in the Python client via
collection.config.delete_vector_index() (weaviate/weaviate-python-client#1991). The CLI
should let operators drive this destructive, asynchronous, irreversible operation and
observe its lifecycle.

Scope

• Add --drop_vector_index <name> to update collection (drops one named vector's index).
• Mutually exclusive with --vector_index; server-version warning (< v1.39.0).
• The drop must run after config.update() (which reads-and-writes the whole schema),
  otherwise a vectorIndexType: "none" vector would be PUT back to the server.
• get collection listing must render a dropped vector as none (and not crash when the
  named vector's vector_index_config is None), showing all distinct index types, e.g.
  hnsw, none.
• Re-issuing the drop on an already-none vector must be allowed (server returns 200):
  it is a no-op while cleanup is in flight and re-enqueues a fresh cleanup task if the
  previous one FAILED — the only operator recovery path for a stalled drop.

Files Involved

• weaviate_cli/defaults.py — UpdateCollectionDefaults.drop_vector_index
• weaviate_cli/commands/update.py — --drop_vector_index option
• weaviate_cli/managers/collection_manager.py — validation, ordering, listing fix
• test/unittests/test_managers/test_collection_manager.py — unit tests
• requirements-dev.txt / setup.cfg — client dependency (temporary pin to PR #1991)
• operating skill docs

Acceptance Criteria

• update collection --drop_vector_index X drops the named vector's index, --json supported
• Rejected when combined with --vector_index
• get collection renders none / hnsw, none and never crashes on a dropped vector
• Re-drop of an already-none vector is accepted (re-trigger), not blocked
• Unit tests cover happy path, validation, and re-trigger
• make lint and make test pass
• Operating skill documentation updated

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🤖 Generated with Claude Code

[jfrancoa]

PR #171 ready for review.