…as_string + precision/scale + named params + utils.exc fix

Five small comparator-parity fixes surfaced by the same audit run.
All are independently small but ship together because they share
the kernel-backend client surface and the audit baseline.

### 1. Async Statement retention

`KernelDatabricksClient.execute_command` closed the parent `Statement`
in `finally` regardless of `async_op`. The kernel's `Statement.close()`
invalidates child handles (see databricks-sql-kernel
`src/statement/validity.rs`), so the async handle died before the user
could poll it — breaking `execute_async` → `is_query_pending` →
`get_async_execution_result`.

Fix: when `async_op=True`, retain the parent Statement in a new
`_async_statements` dict alongside `_async_handles`, and close it
from `close_command`, `close_session`, and `get_execution_result`
after the executed handle is done.

Comparator outcome: STATEMENT_ASYNC suite 3/3 match (was 0/3).

### 2. `intervals_as_string` wire-through

pyarrow's Python bindings cannot decode Arrow's `month_interval` type
(id 21 — `KeyError` from `.as_py`, `to_pylist`, `cast(string)`,
`to_pandas`). Every kernel-backend `SELECT *` over a table with an
`INTERVAL YEAR TO MONTH` column raised `ArrowNotImplementedError` —
32 / 88 audit diffs.

Fix: pass `intervals_as_string=True` to the kernel `Session(...)`
constructor unconditionally. The kernel post-processor stringifies
`Interval` / `Duration` columns server-side to `Utf8` (kernel PR #64).

Comparator outcome: bucket A (ArrowNotImplementedError) 32 → 0.

### 3. Decimal precision/scale extraction

`description_from_arrow_schema` hard-coded `None` for PEP 249
description-tuple slots 4 and 5. For DECIMAL columns the Arrow
schema carries `precision` / `scale` on `Decimal128Type`, but they
were silently dropped — 88 cell diffs (44 precision + 44 scale).

Fix: factor out `_precision_scale_for_arrow_type(arrow_type)` and
call it from the description builder. Today it only handles
decimals; future extensions slot in here.

Comparator outcome: 88 precision+scale diffs → 0.

### 4. Named-parameter binding

`bind_tspark_params` raised `NotSupportedError` for any
`TSparkParameter` with `name` set. The canonical SEA proto marks
`StatementParameter.name` as `openapi_required=true` (named is the
spec-required public form; `ordinal` is `PUBLIC_UNDOCUMENTED`).
Kernel PR #65 added a `Statement.bind_named_param` PyO3 API.

Fix: route named bindings via the new API. Positional ordinals
self-increment so a named entry in the middle of the list doesn't
claim a positional slot.

Comparator outcome: PREPARED_STATEMENT_NAMED 1/1 match (was 0/1).
Full thrift-vs-kernel run: 97/132 match (was 96/132).

### 5. `utils.ParamEscaper` `exc.ProgrammingError` import fix

`ParamEscaper.escape_args` and `escape_item` both raised
`exc.ProgrammingError(...)`, but `exc` was never imported. On any
unsupported parameter shape the user saw `NameError: name 'exc' is
not defined` instead of a clean PEP 249 `ProgrammingError`.

Surfaced via the same audit harness running INLINE_PARAMS: both
backends raised `NameError`, which the comparator's class+message
match treated as parity — a false-positive that hid both the driver
bug and the underlying caller-side type mismatch.

Fix: import `ProgrammingError` directly from `databricks.sql.exc`
(matching the pattern used in `session.py`, `client.py`,
`result_set.py`, etc.) and replace the two `exc.ProgrammingError(...)`
sites with bare `ProgrammingError(...)`.

## Dependencies

Items #2, #3, and #4 require the matching databricks-sql-kernel
changes: PR #64 (`intervals_as_string` + empty-result schema fix)
and PR #65 (named-param binding). For local testing the comparator
harness uses `KERNEL_FREEZE=1` against a kernel checkout of the
feature branches.

## Headline comparator results (full thrift-vs-kernel run)

|                                    | Before | After |
|------------------------------------|--------|-------|
| match / diff / skipped             | 60 / 88 / 0 | **97 / 34 / 1** |
| Bucket A (ArrowNotImplementedError)| 32     | 0     |
| `decimal_column` precision/scale   | 88     | 0     |
| Bucket B1 (named params)           | 1      | 0     |
| Suites fully clean                 | 12 / 30 | **17 / 30** |

Remaining 34 diffs cluster into documented causes (complex types in
`fetchall_arrow`, timestamp tz on Arrow path, VOID surface,
METADATA pattern semantics) — tracked in
`~/docs/python-kernel/comparator-diff-tasklist.md`.

## Test plan

- [x] Manual repro of async path: `cur.execute_async("SELECT 1")` →
      `is_query_pending` → `get_async_execution_result` → `fetchall`
      succeeds on `use_kernel=True`
- [x] Manual repro of interval path: `cur.execute("SELECT ym_interval_column FROM ...")`
      returns string-shaped rows matching the Thrift surface
- [x] Manual repro of decimal path: `cur.description` for a
      `DECIMAL(10,2)` column now reports `precision=10, scale=2`
- [x] Live e2e: `test_parameterized_query_named_params` /
      `test_parameterized_query_named_param_with_null` (added)
- [x] Unit tests: `bind_tspark_params` named/positional/mixed
      routing (`tests/unit/test_kernel_type_mapping.py`)
- [x] Manual repro of utils fix:
      `ParamEscaper().escape_args(object())` raises
      `ProgrammingError`, not `NameError`
- [x] Full thrift-vs-kernel comparator: 97 / 34 / 1 of 132
- [x] Existing connector unit suite: 752 passed

Co-authored-by: Isaac
Signed-off-by: Vikrant Puppala <vikrant.puppala@databricks.com>