Why custom Dialect doesn't work properly

I have extended a Dialect named MyDialect, and implemented the is_identifier_start method. is_identifier_start allows the identifier to start with a number, but why does my test code report an error? The prompt says that it cannot start with a number, and if I run it with the native HiveDialect, the code No problem again

**My Code：**
```
#[derive(Debug)]
pub struct MyDialect;

impl Dialect for MyDialect {
fn is_delimited_identifier_start(&self, ch: char) -> bool {
(ch == '"') || (ch == '`')
}

fn is_identifier_start(&self, ch: char) -> bool {
ch.is_ascii_lowercase() || ch.is_ascii_uppercase() || ch.is_ascii_digit() || ch == '$'
}

fn is_identifier_part(&self, ch: char) -> bool {
ch.is_ascii_lowercase()
|| ch.is_ascii_uppercase()
|| ch.is_ascii_digit()
|| ch == '_'
|| ch == '$'
|| ch == '{'
|| ch == '}'
}

fn supports_filter_during_aggregation(&self) -> bool {
true
}
}

#[cfg(test)]
mod tests {
use sqlparser::parser::Parser;

#[test]
pub fn test_ast() {
let sql = "SELECT * from 1_a";
let expected_sql = "SELECT * from 1_a"
.replace("\n", "");
let dialect = crate::core::ast::MyDialect {};
let ast = Parser::parse_sql(&dialect, sql).unwrap();
assert_eq!(ast[0].to_string(), expected_sql);
}
}
```

**HIveDialect Code（from sqlparser-rs）**
use crate::dialect::Dialect;

/// A [`Dialect`] for [Hive](https://hive.apache.org/).
```
#[derive(Debug)]
pub struct HiveDialect {}

impl Dialect for HiveDialect {
fn is_delimited_identifier_start(&self, ch: char) -> bool {
(ch == '"') || (ch == '`')
}

fn is_identifier_start(&self, ch: char) -> bool {
ch.is_ascii_lowercase() || ch.is_ascii_uppercase() || ch.is_ascii_digit() || ch == '$'
}

fn is_identifier_part(&self, ch: char) -> bool {
ch.is_ascii_lowercase()
|| ch.is_ascii_uppercase()
|| ch.is_ascii_digit()
|| ch == '_'
|| ch == '$'
|| ch == '{'
|| ch == '}'
}

fn supports_filter_during_aggregation(&self) -> bool {
true
}
}
```

**Error：**

Start at Parser::parse_sql and the Dialect trait, then compare the custom implementation with HiveDialect using the provided test_ast reproduction for SELECT * from 1_a. Trace how identifier-start handling is applied and confirm the expected parsing behavior; done means the reproduction is explained or the parser behavior is corrected with a regression test.

Written by the indexing model from the issue text.