Function is_bigserial_default 

fn is_bigserial_default(default_value_expression: &str) -> bool

Checks if a given default value expression is a BIGSERIAL default.

Normally, all unsupported expressions should fail in from_text parsing. However, we make a special exception for nextval() function because:

1. When users set a column as BIGSERIAL (usually as primary key), PostgreSQL automatically generates a default value expression like nextval('sequence_name'::regclass)
2. This is a very common scenario in real-world usage
3. For existing columns with nextval() default, we skip parsing the default value to avoid schema change failures

TODO: In the future, if we can distinguish between newly added columns and existing columns, we should modify this logic to:

• Skip default value parsing for existing columns with nextval()
• Report error for newly added columns with nextval() (since they should be handled differently)