MsSqlQueueBuilder.GetExistsQuery formats both names into SQL string literals

`src/Paramore.Brighter.MessagingGateway.MsSql/MsSqlQueueBuilder.cs:84`:

```csharp
public static string GetExistsQuery(string queueTableName, string schemaName = "dbo") =>
string.Format(QUEUE_EXISTS_SQL, queueTableName, schemaName);
```

and the template it formats into:

```sql
WHERE t.name = '{0}' AND s.name = '{1}'
```

Both names land inside SQL string literals with no escaping, so a name containing a single quote changes the statement. `GetDDL` and `GetIndexDDL` have the same shape into `CREATE TABLE [{0}]`, where the character that matters is `]` rather than `'`.

### Why this is worth fixing rather than guarding at each call site

`MsSqlQueueBuilder` is **public API** — deliberately so, because the MSSQL gateway had no provisioning path and callers were expected to run the DDL themselves. So the guard cannot live in a caller without every caller re-deriving it, and each one getting it slightly differently.

A worked example of getting it wrong: while moving the sample's provisioner into the gateway (#4343) I ported a guard of `^[A-Za-z_][A-Za-z0-9_]{0,127}$`, which looks careful and is wrong — a **bracketed** identifier legally holds hyphens, and this repository's own test suite names queue tables `queue_test_`. The existing suite caught it. That is the argument for one guard, in the builder, next to the `string.Format`.

### Suggested fix

- `GetExistsQuery`: bind the two names as parameters rather than formatting them, or at minimum escape `'` as `''`.
- `GetDDL` / `GetIndexDDL`: escape `]` as `]]`, which is SQL Server's own convention for a bracketed identifier, and reject a name over 128 characters.

A parameterised existence check also fixes a second, quieter problem: `GetExistsQuery` defaults `schemaName` to a literal `dbo`, while every statement the gateway issues against the queue is **unqualified** and so resolves through the caller's default schema. A login whose default schema is not `dbo` gets an existence check that disagrees with the runtime. #4343 sidesteps this by asking `SCHEMA_NAME()`, but `GetExistsQuery` is public and still carries the `dbo` default.

Raised in review of #4331; #4343 is the first production caller of this family of methods.

Start in src/Paramore.Brighter.MessagingGateway.MsSql/MsSqlQueueBuilder.cs, focusing on GetExistsQuery, GetDDL, and GetIndexDDL, then inspect the existing tests and the production caller from #4343. Verify behavior for quoted names, bracketed identifiers, names over 128 characters, and schemas other than dbo; done means generated SQL remains valid and existence checks match the gateway's schema resolution.

Written by the indexing model from the issue text.