Test coverage for the =~ regex match operator and is_regex_match builtin was minimal, missing key behavioral cases. Adds the following test cases:

=~ operator tests:

• regex_none_input – None =~ pattern returns false
• regex_markdown_match – matching Markdown text node returns Markdown(Text("true"))
• regex_markdown_non_match – non-matching Markdown text node returns Markdown(Text("false"))
• regex_in_if – =~ used as a conditional predicate: "test" | if (. =~ "test"): true else: false
• regex_invalid_pattern – malformed regex (e.g. "[invalid") propagates an error

is_regex_match builtin tests:

• is_regex_match_match – is_regex_match("test1", "[a-z0-9]+") returns true
• is_regex_match_no_match – is_regex_match("abc", "[0-9]+") returns false
• is_regex_match_none_input – is_regex_match(., pattern) with None input returns false
• is_regex_match_markdown – Markdown text node input returns Markdown(Text("true"))
• is_regex_match_invalid_pattern – malformed regex propagates an error

Note: Markdown input cases return Markdown(Text("true"/"false")) rather than Boolean, consistent with how the mq engine wraps non-Markdown results when the input is a Markdown node (same behavior as matches_url).

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