fix(query): support IF NOT EXISTS for ALTER TABLE ADD COLUMN

[sundy-li]

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Summary

• add parser support for ALTER TABLE ... ADD COLUMN IF NOT EXISTS
• make duplicate add-column requests a no-op when IF NOT EXISTS is present
• add Rust and SQL regressions for the new idempotent DDL behavior

Changes

1. Extended the ALTER TABLE add-column AST, display formatting, and parser grammar with an if_not_exists flag.
2. Threaded the new flag through the add-column plan in the SQL binder.
3. Short-circuited binder/interpreter execution when the target column already exists and IF NOT EXISTS is specified.
4. Added AST parser coverage, a fuse storage integration test, and a sqllogictest regression.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Pair with the reviewer to explain why

Validation commands:

cargo test -p databend-common-ast --test it parser::test_statement -- --exact
cargo test -p databend-query --test it add_column_if_not_exists -- --nocapture
cargo clippy -p databend-common-sql --lib -- -D warnings
cargo clippy -p databend-common-ast --test it -- -D warnings

Type of change

• Bug fix

Fixes #19611

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[sundy-li]

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