fix(collection): preserve writing segment on DDL failure

[egolearner]

Summary

• Keep the live writing segment unchanged while CreateIndex/DropIndex persisted-segment tasks are still running.
• Switch non-empty writing segments under the old schema before DDL task execution.
• Create the new-schema writing segment only after DDL tasks succeed, and make it live only after the new manifest is flushed.
• Add Python regression coverage for FTS and inverted drop_index reopen consistency.

Closes #573

Tests

• ninja -C build.release _zvec
• build.release/bin/collection_test --gtest_filter="CollectionTest.Feature_DropAndRecreateScalarIndex*:CreateIndex:DropIndex"
• python -m pytest -o addopts="" python/tests/detail/test_collection_ddl.py::TestIndexDDL::test_drop_index_reopen_after_persisted_task_result -q