examples: replace sql_dialect with custom_sql_parser example (#19383)

## Which issue does this PR close?

Related to #16756

## Rationale for this change

The existing `sql_dialect.rs` example demonstrates `COPY ... STORED AS
...`, which is actually already fully supported by the standard
`DFParser`.

This PR replaces it with the example from #16756: `CREATE EXTERNAL
CATALOG ... STORED AS ... LOCATION ...` with automatic table discovery.

## What changes are included in this PR?

The first commit updates `dialect.rs` to show that `DFParser` already
handles `COPY ... STORED AS`, making it clear this syntax doesn't need
customization.

Example output from `cargo run --example sql_ops -- dialect`:

```
Query: COPY source_table TO 'file.fasta' STORED AS FASTA
--- Parsing without extension ---
Standard DFParser: Parsed as Statement::CopyTo: COPY source_table TO file.fasta STORED AS FASTA

--- Parsing with extension ---
Custom MyParser: Parsed as MyStatement::MyCopyTo: COPY source_table TO 'file.fasta' STORED AS FASTA
```

The second commit adds a new `custom_sql_parser.rs` example that
implements `CREATE EXTERNAL CATALOG my_catalog STORED AS <format>
LOCATION '<url>'` with automatic table discovery from object storage. It
also removes the old `dialect.rs` example.

## Are these changes tested?

Yes, the new example is runnable with `cargo run --example sql_ops --
custom_sql_parser` and demonstrates the full flow from parsing custom
DDL through registering the catalog to querying discovered tables.

Example output:

```
=== Part 1: Standard DataFusion Parser ===

Parsing: CREATE EXTERNAL CATALOG parquet_testing
         STORED AS parquet
         LOCATION 'local://workspace/parquet-testing/data'
         OPTIONS (
           'schema_name' = 'staged_data',
           'format.pruning' = 'true'
         )

Error: SQL error: ParserError("Expected: TABLE, found: CATALOG at Line: 1, Column: 17")

=== Part 2: Custom Parser ===

Parsing: CREATE EXTERNAL CATALOG parquet_testing
         STORED AS parquet
         LOCATION 'local://workspace/parquet-testing/data'
         OPTIONS (
           'schema_name' = 'staged_data',
           'format.pruning' = 'true'
         )

  Target Catalog: parquet_testing
  Data Location: local://workspace/parquet-testing/data
  Resolved Schema: staged_data
  Registered 69 tables into schema: staged_data
Executing: SELECT id, bool_col, tinyint_col FROM parquet_testing.staged_data.alltypes_plain LIMIT 5

+----+----------+-------------+
| id | bool_col | tinyint_col |
+----+----------+-------------+
| 4  | true     | 0           |
| 5  | false    | 1           |
| 6  | true     | 0           |
| 7  | false    | 1           |
| 2  | true     | 0           |
+----+----------+-------------+
```

## Are there any user-facing changes?

Documentation only. I replaced the `sql_dialect.rs` example with
`custom_sql_parser.rs` and updated the README. No API changes.

Author: geoffreyclaude

datafusion-examples/README.md

@@ -191,12 +191,12 @@ cargo run --example dataframe -- dataframe
 
 #### Category: Single Process
 
-| Subcommand | File Path                                             | Description                    |
-| ---------- | ----------------------------------------------------- | ------------------------------ |
-| analysis   | [`sql_ops/analysis.rs`](examples/sql_ops/analysis.rs) | Analyze SQL queries            |
-| dialect    | [`sql_ops/dialect.rs`](examples/sql_ops/dialect.rs)   | Implement a custom SQL dialect |
-| frontend   | [`sql_ops/frontend.rs`](examples/sql_ops/frontend.rs) | Build LogicalPlans from SQL    |
-| query      | [`sql_ops/query.rs`](examples/sql_ops/query.rs)       | Query data using SQL           |
+| Subcommand        | File Path                                                               | Description                                        |
+| ----------------- | ----------------------------------------------------------------------- | -------------------------------------------------- |
+| analysis          | [`sql_ops/analysis.rs`](examples/sql_ops/analysis.rs)                   | Analyze SQL queries                                |
+| custom_sql_parser | [`sql_ops/custom_sql_parser.rs`](examples/sql_ops/custom_sql_parser.rs) | Implement a custom SQL parser to extend DataFusion |
+| frontend          | [`sql_ops/frontend.rs`](examples/sql_ops/frontend.rs)                   | Build LogicalPlans from SQL                        |
+| query             | [`sql_ops/query.rs`](examples/sql_ops/query.rs)                         | Query data using SQL                               |
 
 ## UDF Examples