Add getValidReactions function to compute valid reaction list dynamically

[Copilot]

This PR addresses feedback from #2241 by refactoring the reaction validation logic to eliminate code duplication.

Changes

Previously, the valid reaction values were defined in two places:

1. Inside the isValidReaction() function as a local map
2. Hardcoded as a string slice in the error message in compiler.go

This PR refactors the code to:

• Extract validReactions map to package level in reactions.go
• Add getValidReactions() function that dynamically computes the list of valid reactions from the map keys
• Update compiler.go to use getValidReactions() instead of the hardcoded list

Benefits

• Single source of truth: Valid reactions are now defined in one place
• Maintainability: Adding or removing reaction types only requires updating the map
• Consistency: Error messages automatically reflect the current set of valid reactions

Example

// Before: validReactions was a local variable
func isValidReaction(reaction string) bool {
    validReactions := map[string]bool{...}
    return validReactions[reaction]
}

// After: validReactions is package-level, with a getter function
var validReactions = map[string]bool{...}

func getValidReactions() []string {
    reactions := make([]string, 0, len(validReactions))
    for reaction := range validReactions {
        reactions = append(reactions, reaction)
    }
    return reactions
}

Fixes feedback in #2241

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