[WIP] refactor: finalize db_case_config at a single choke point

[jamesgao-jpg]

What

Unifies every post-construction modification of db_case_config into a
single choke point, so a case config is produced in exactly one place and
only read afterwards.

New vectordb_bench/backend/db_case_config.py:

• finalize_db_case_config(db, case_type, base_config, *, parameters, dataset)
  composes the two existing post-construction transforms:
  1. CLI-only FTS routing (moved from select_cli_db_case_config in
     cli/cli.py: vector config → backend FTS config with compatible-field
     copy, plus BM25 overrides);
  2. dataset-derived metric_type (previously applied by mutating
     task.db_case_config in assembler.py), now via model_copy so the
     input config is never mutated.
• Both task entry points call it: cli.run() (with the CLI parameters dict)
  and the web UI generate_tasks() (without). The assembler no longer
  mutates the config; select_cli_db_case_config remains as a thin
  backward-compatible wrapper.

Why

db_case_config was touched in four layers (CLI construction, run()
FTS routing, assembler in-place mutation, task-runner/client reads), and the
CLI-only routing was skipped entirely on the web UI path. This made changes
to config handling easy to break and the two entry points could diverge.
Routing both entry points through one resolver (and deleting the in-place
mutation) removes that class of bug.

Behavior / compatibility

• No behavior change: FTS-routing semantics, the metric_type default rule,
  and both entry-point results are preserved. Covered by the new resolver
  tests plus the existing FTS-routing tests, which now exercise the same
  code through the compatibility wrapper.
• Cross-PR note: this refactor moves the FTS-compatible field whitelist
  (including force_merge_target_size_mb) into the resolver, so it subsumes
  the one-line whitelist fix in PR feat: add CLI control for Milvus force-merge size and toggle #871 (cli/cli.py). When both merge, drop
  feat: add CLI control for Milvus force-merge size and toggle #871's cli/cli.py hunk — the field already lives in the resolver.

Testing (Level 2)

• Affected component tests on the remote client worktree (origin/main base):
  tests/test_db_case_config.py (8 new), test_fts_cli_user_control.py,
  test_oss_opensearch_fts.py, test_milvus.py, test_milvus_zilliz_cli.py
  → 53 passed, 1 integration deselected.
• Whole-repo ruff check → All checks passed; black --check → 230
  files unchanged (both match the main baseline).

Follow-up (separate PR)

frozen=True enforcement across all case-config classes (~55 root classes in
44 backend config files), reworking the self-assigning cached-field methods
in aws_opensearch/oss_opensearch parse_metric(), hologres, and
pgvector, with a cross-backend regression sweep.

[sre-ci-robot]

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