Add support for DuckDB LAMBDA keyword syntax

[lovasoa]

Summary

This PR adds support for parsing DuckDB's LAMBDA keyword syntax for lambda functions.

DuckDB supports two syntaxes for lambda functions:

• Arrow syntax: x -> x + 1 (already supported, but deprecated in duckdb)
• Lambda keyword syntax: lambda x : x + 1 (preferred syntax in duckdb, added in this PR)

Initially reported in sqlpage/SQLPage#1171

Changes

• Added LAMBDA keyword to keywords list
• Added LambdaSyntax enum to track which syntax style was used (for correct round-tripping)
• Added parse_lambda_expr() to parse lambda <params> : <expr> syntax
• Updated LambdaFunction struct to include syntax field
• Added tests for DuckDB lambda functions

Examples

-- Single parameter
SELECT list_filter([1, 2, 3], lambda x : x > 1)

-- Multiple parameters (with index)
SELECT list_filter([1, 3, 1, 5], lambda x, i : x > i)

-- Method call syntax
SELECT [3, 4, 5, 6].list_filter(lambda x : x > 4)

Test plan

• Added test_duckdb_lambda_function test covering single param, multi-param, and method call syntax
• All existing lambda tests pass (Databricks, ClickHouse, common)
• All DuckDB tests pass

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🤖 Generated with Claude Code and and reviewed by me, @lovasoa, a human.

[lovasoa]

Hi @iffyio, @alamb ! Do you think you could have a look at this ? The old duckdb lambda syntax is deprecated, so I suppose this is going to pop out more and more. This comes from a bug report in sqlpage.