Fix F# option date index member selection

[nkosi23]

Motivation

• Fix a regression where F# option date/time fields were resolved as generic value types and emitted CAST(... as timestamp)/timestamptz in computed index DDL instead of using the mt_immutable_* functions, which caused Postgres errors about non-immutable functions in index expressions.
• Ensure the computed index test exercises F# option value type registration so the regression is covered.

Description

• In src/Marten/StoreOptions.MemberFactory.cs added a guard in ValueTypeMemberSource.TryResolve to skip treating FSharpOption<T> as a generic value type when T is DateTime, DateTimeOffset, DateOnly, or TimeOnly, by returning false early so the specialized DateTimeMember/DateTimeOffsetMember logic is used instead.
• Added a helper method isSpecialFSharpOptionDateType(Type) to centralize the date/time type check.
• In src/DocumentDbTests/Indexes/computed_indexes.cs updated the test fsharp_date_related_options_indexes_are_created to call _.RegisterFSharpOptionValueTypes() so the F# option value types are registered during test setup.

Testing

• No automated test suite was executed as part of this change (tests were not run).
• The fsharp_date_related_options_indexes_are_created test was updated to register F# option value types to cover the regression scenario expected to be validated by the test suite.

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Codex Task