Per-table limit on total size of non-key columns (2000 MiB) is not enforced

The emulator does not enforce Cloud Spanner's per-table limit on the total size of non-key columns (2,097,152,000 bytes = 2000 MiB). DDL that exceeds the limit succeeds on the emulator but fails on Cloud Spanner, in all three paths: `CREATE TABLE`, `ALTER TABLE ... ADD COLUMN`, and `ALTER TABLE ... ALTER COLUMN`.

Per the [quotas documentation](https://docs.cloud.google.com/spanner/quotas#tables):
> Total size of non-key columns | 2,000 MiB

### Reproduction

Image: `gcr.io/cloud-spanner-emulator/emulator:latest` (digest `sha256:ad54472fe7b161b9214f7f816f304b649a4779e348229c375ac067f5ed5a6422`, corresponds to `v1.5.55`).

Setup — two tables whose non-key columns are at or under the limit. Both the emulator and Cloud Spanner accept these (200 × `STRING(MAX)` is exactly 2,097,152,000 bytes, so the boundary itself is fine):

```sql
-- Exactly at the limit: 200 STRING(MAX) columns
CREATE TABLE T200 (
Id STRING(36) NOT NULL,
Col001 STRING(MAX),
-- ... 200 STRING(MAX) columns in total ...
Col200 STRING(MAX),
) PRIMARY KEY (Id);

-- Under the limit: 199 STRING(MAX) columns + 2 STRING(100) columns
CREATE TABLE TAlter (
Id STRING(36) NOT NULL,
Col001 STRING(MAX),
-- ... 199 STRING(MAX) columns in total ...
Col199 STRING(MAX),
Extra1 STRING(100),
Extra2 STRING(100),
) PRIMARY KEY (Id);
```

Statements that exceed the limit:

```sql
-- (A) CREATE TABLE with 201 STRING(MAX) columns
CREATE TABLE T201 (
Id STRING(36) NOT NULL,
Col001 STRING(MAX),
-- ... 201 STRING(MAX) columns in total ...
Col201 STRING(MAX),
) PRIMARY KEY (Id);

-- (B) Add a column to the at-limit table
ALTER TABLE T200 ADD COLUMN Col201 STRING(MAX);

-- (C) Widen an existing column
ALTER TABLE TAlter ALTER COLUMN Extra1 STRING(MAX);
```

### Expected (Cloud Spanner behavior)

All three statements fail:

```
(A) ERROR: Total size of non-key columns in Table T201 2107637760 exceeds the limit of 2097152000.
(B) ERROR: Cannot add column Col201. Total size of non-key columns in table T200 2107637760 exceeds the limit of 2097152000.
(C) ERROR: Cannot increase length of column Extra1. Total size of non-key columns in table TAlter is 2097152400 which exceeds the limit of 2097152000.
```

### Actual (Emulator behavior)

All three statements succeed, and the resulting schema contains the over-limit tables (`T201`, `T200` with `Col201`, and `TAlter` with `Extra1 STRING(MAX)`).

### Impact

CI that runs DDL against the emulator passes, but the same DDL fails when applied to Cloud Spanner — typically discovered only at migration time, when a wide table (e.g. one with many `STRING(MAX)` columns) crosses the 2000 MiB budget.

Start by tracing the emulator's DDL validation for CREATE TABLE, ALTER TABLE ... ADD COLUMN, and ALTER TABLE ... ALTER COLUMN. Reproduce the three over-limit statements and compare them with the accepted at-limit cases, then add coverage showing that each path rejects totals above 2,097,152,000 bytes while accepting the boundary.

Written by the indexing model from the issue text.