feat(isthmus): map REGEXP_EXTRACT to Substrait regexp_match_substring

[nielspardon]

What

Maps Calcite's BigQuery-library REGEXP_EXTRACT(value, regexp) operator to Substrait's two-argument regexp_match_substring(input, pattern).

Why

REGEXP_EXTRACT had no entry in FunctionMappings, so SQL using it failed to convert to Substrait (Unable to convert call REGEXP_EXTRACT(...)). The two-argument form lines up directly with the two-argument regexp_match_substring impl in functions_string.yaml.

Scope / notes

• Two-argument form only. Calcite's optional position/occurrence args (3–4 arg forms) are not handled — Substrait has no 3/4-arg impl, so those would need argument padding to the 5-arg impl and are left for a follow-up.
• Options (case_sensitivity, multiline, dotall) are defaulted automatically by the function matcher, the same way substring's negative_start option is.
• Semantic caveat: BigQuery REGEXP_EXTRACT returns capture group 1 when the pattern contains a capturing group; the 2-arg Substrait impl returns the full match. Identical for group-less patterns, divergent otherwise. Called out in a code comment.
• AutomaticDynamicFunctionMappingRoundtripTest had used regexp_match_substring as an example of an unmapped function. Since it is now mapped, the test is repointed at the still-unmapped regexp_count_substring.

Testing

• New round-trip test in StringFunctionTest over c16/vc32/vc.
• StringFunctionTest, AutomaticDynamicFunctionMappingRoundtripTest, and FunctionConversionTest pass locally.

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