fix(ast): avoid round-trip parser panic for nested joins

[sundy-li]

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Fixes #19571

Summary

• stop the debug-only SQL round-trip check from panicking when nested join grouping cannot be reparsed from the pretty-printed SQL
• keep the original parse result and emit a warning instead, so parser quality checks do not crash the server
• add a regression test for grouped nested joins with separate join conditions

Changes

1. Update src/query/ast/src/parser/parser.rs so the debug round-trip validation returns an error message instead of panicking/asserting.
2. Log the round-trip failure with log::warn! and keep parse_sql() returning the original parsed statement.
3. Add integration coverage in src/query/ast/tests/it/display.rs for nested joins that currently lose grouping when displayed.

Tests

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

Validation:

• cargo fmt --all --check
• cargo test -p databend-common-ast
• cargo clippy -p databend-common-ast --all-targets -- -D warnings

Type of change

• Bug fix

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This change is