[Repo Assist] [Beam] Fix String.IndexOf/LastIndexOf with StringComparison argument

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🤖 This is an automated draft PR created by Repo Assist.

Summary

• Bug: str.IndexOf(sub, StringComparison.Ordinal) in the Beam target was matching the generic [ sub; startIdx ] pattern. The StringComparison enum integer value (e.g. 4 for Ordinal) was being passed as a start index to fable_string:index_of/3, producing wrong results.
• Fix: Add StringComparisonEnumValue-specific patterns for IndexOf and LastIndexOf before the generic two-arg patterns, so the StringComparison argument is stripped (as in the JS/TS target) and only the substring (and optional start index) is passed to the Erlang library function.
• Tests: Adds four new tests to tests/Beam/StringTests.fs, mirroring the existing JS tests for this overload.

Root Cause

Pattern matching in Beam/Replacements.fs did not distinguish between startIdx: int and StringComparison: enum when both are passed as the second argument to IndexOf/LastIndexOf. F# pattern matching uses first-match-wins, so the [ sub; StringComparisonEnumValue ] patterns must appear before the [ sub; startIdx ] wildcard patterns.

Limitations

Like the JS/TS target, this fix does not implement case-insensitive comparison for StringComparison.OrdinalIgnoreCase — it strips the comparison argument and uses Beam's default binary (ordinal) comparison. This matches the existing behavior of all other Fable targets.

Test plan

• Beam tests pass: ./build.sh test beam
• No regressions in other targets

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