Add parser support for aggregate functions

[writemorecode]

Adds parser support for aggregate functions: SUM, AVG, STDDEV, MIN, MAX, COUNT.

[Copilot]

Pull Request Overview

This PR adds parser support for aggregate functions (SUM, AVG, STDDEV, MIN, MAX, COUNT) to the SQL parser. Key changes include updating test files to cover aggregate functions, extending the parser to handle new aggregate function tokens, and updating token kind definitions to support aggregate-specific keywords.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/databas_sql_parser/tests/parse_select.rs	Updated test import to support the new AggregateFunction variant
crates/databas_sql_parser/tests/parse_aggregate.rs	Added new tests covering all aggregate functions
crates/databas_sql_parser/src/parser/mod.rs	Introduced parse_aggregate_function and integrated it into expression parsing
crates/databas_sql_parser/src/parser/expr.rs	Added AggregateFunction variant to the Expression enum and Display implementation
crates/databas_sql_parser/src/lexer/token_kind.rs	Extended Keyword enum and token parsing to recognize aggregate keywords

Comments suppressed due to low confidence (2)

crates/databas_sql_parser/tests/parse_aggregate.rs:12

• Consider adding negative tests to verify that incorrect aggregate function syntax (e.g. missing parentheses or unsupported tokens) is handled properly by the parser.

fn test_all_aggregate_functions() {

crates/databas_sql_parser/src/parser/mod.rs:166

• [nitpick] The parameter name 'agg' could be made more descriptive (for example, 'aggregate_token') to improve code readability.

TokenKind::Keyword(Keyword::Aggregate(agg)) => self.parse_aggregate_function(agg)?,