ReplOptionAttribute.CaseSensitivity is unsettable (nullable enum) — per-option case override is dead via attribute

[carldebilly]

Summary

ReplOptionAttribute.CaseSensitivity is typed ReplCaseSensitivity? (nullable enum). A nullable enum cannot be a named attribute argument in C#, so any attempt to set it fails to compile:

[ReplOption(CaseSensitivity = ReplCaseSensitivity.CaseInsensitive)] string? channel
// error CS0655: 'CaseSensitivity' is not a valid named attribute argument
// because it is not a valid attribute parameter type

The property therefore always resolves to null through the attribute path. OptionSchemaBuilder feeds it into every option entry as CaseSensitivity: optionAttribute?.CaseSensitivity, so the per-option case-sensitivity override the property is meant to provide is effectively dead code for options declared via [ReplOption].

(The OptionSchemaEntry.CaseSensitivity field itself works — it is exercised by direct-schema tests and by value-alias / enum-flag configuration, which carry their own CaseSensitivity. Only the option-level override via the attribute is unreachable.)

Impact

• Users cannot make a single option case-insensitive (or case-sensitive) independently of the global ParsingOptions.OptionCaseSensitivity.
• Discovered while reviewing fix(autocomplete): option-value & option-region completion parity edges #55: the "ambiguous pending option token" scenario (a token matching both a CaseInsensitive --Foo and a --foo) is not reachable precisely because this override cannot be set.

Options

1. Make it settable. Attribute-legal alternatives:
   • Replace the nullable enum with two bool flags (e.g. CaseInsensitive / and rely on absence for "inherit"), or a non-nullable enum with an explicit Inherit member (ReplCaseSensitivity would need such a member, or a dedicated ReplOptionCaseSensitivity { Inherit, CaseSensitive, CaseInsensitive }).
   • Expose the override through a fluent builder API instead of (or in addition to) the attribute.
2. Remove it. If per-option case override is not a supported feature, drop the property and the optionAttribute?.CaseSensitivity plumbing so the API does not advertise a knob that cannot be used.

Either way, add a test that actually sets the override and asserts a differently-cased token resolves (or does not) accordingly — today no test can exercise the attribute path.

Context

• src/Repl.Core/Parameters/Attributes/ReplOptionAttribute.cs — the property.
• src/Repl.Core/Internal/Options/OptionSchemaBuilder.cs — consumes optionAttribute?.CaseSensitivity (multiple sites).
• Found during the fix(autocomplete): option-value & option-region completion parity edges #55 review (relates to the ambiguous-token finding, deemed unreachable for this reason).