feat(rust): add metadata FFI layer with streaming Arrow pass-through (#281)

## Summary

- Adds a `metadata-ffi` feature flag exposing `extern "C"` functions for
catalog metadata operations (GetCatalogs, GetSchemas, GetTables,
GetColumns, GetPrimaryKeys, GetForeignKeys)
- Results stream as Arrow `RecordBatchReader` via the Arrow C Data
Interface (`FFI_ArrowArrayStream`), using `ResultReaderAdapter` to
bridge `ResultReader` → `RecordBatchReader` without buffering
- Includes `ConnectionMetadataService` (thin pass-through to
`DatabricksClient`), C FFI handle/error management with `catch_unwind`
panic safety, and thread-local error buffer
- `list_columns` accepts optional catalog — uses `SHOW COLUMNS IN ALL
CATALOGS` server-side when catalog is None/wildcard (no client-side
multi-catalog orchestration)

## Files

### New
- `ffi/mod.rs`, `ffi/catalog.rs`, `ffi/error.rs`, `ffi/handle.rs` — C
FFI surface (9 extern "C" functions)
- `metadata/service.rs` — `ConnectionMetadataService` returning
streaming `Box<dyn ResultReader>`
- `spec/odbc-metadata-ffi-design.md` — Design specification

### Modified
- `client/mod.rs` — `list_columns` catalog: `&str` → `Option<&str>`
- `client/sea.rs` — Handle optional catalog in `list_columns`
- `metadata/sql.rs` — `build_show_columns` supports ALL CATALOGS,
returns `String` (not `Result`)
- `connection.rs` — Updated `list_columns` call sites
- `reader/mod.rs` — Added `ResultReaderAdapter` unit tests

## Data Flow

```
DatabricksClient.list_*() → ExecuteResult.reader → ConnectionMetadataService (pass-through)
  → export_reader() → ResultReaderAdapter → FFI_ArrowArrayStream → C caller
```

No intermediate `collect_batches` or `concat_batches` — batches stream
lazily from network to caller.

## Test plan
- [x] All 125 unit tests pass (`cargo test`)
- [x] `cargo clippy -- -D warnings` clean
- [x] `cargo fmt` clean
- [ ] Integration test with ODBC wrapper consuming FFI functions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: gopalldb

rust/Cargo.lock

@@ -64,6 +64,9 @@ async-trait = "0.1"
 # Error handling
 thiserror = "2"
 
+# Arrow C Data Interface for metadata FFI
+arrow = { version = "57", optional = true, default-features = false, features = ["ffi"] }
+
 [dev-dependencies]
 tokio = { version = "1", features = ["rt-multi-thread", "macros"] }
 wiremock = "0.6"
@@ -72,6 +75,7 @@ adbc_driver_manager = "0.22"
 [features]
 default = []
 ffi = ["adbc_ffi"]
+metadata-ffi = ["ffi", "dep:arrow"]
 
 [lib]
 name = "databricks_adbc"