fix(bash): BASH_REMATCH not populated when regex starts with parens

[chaliy]

Summary

• Fix parser's parse_conditional to check saw_regex_op flag when encountering LeftParen token inside [[ ]]
• Regex patterns starting with capture groups (e.g. ([0-9]+)) were incorrectly parsed as grouping parens instead of regex content
• Adds 6 test cases covering basic match, multiple groups, nested groups, no-match clearing, and conditional usage

Test plan

• All 6 new regex test cases pass
• cargo test --all-features passes
• bash_comparison_tests pass (validated against real bash)
• cargo clippy and cargo fmt clean