Address Robert's review comments on variant casing

[sffc]

This PR addresses Robert's review comments on PR #8084.

Instead of converting back and forth between BCP-47 lowercase and CLDR uppercase in the provider using closures, we now normalize the variant keys to lowercase in the CLDR JSON deserialization step in cldr_serde. This allows us to simplify the provider implementation and completely revert the macro changes from the first PR, as the key/attribute transforms are no longer needed.

🤖 This pull request was created by an AI agent working with @sffc.

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[sffc]

🤖 This comment is written by an AI agent working with @sffc.

We cannot simply use Variant::try_from_str here because some keys in CLDR might contain alt variant suffixes (e.g., -alt-variant), which are not valid BCP-47 variants as a whole and would fail to parse, causing deserialization to fail.

If we wanted to use Variant for normalization, we would need to split the key, normalize the variant part, and reconstruct it. Here is how we could handle it:

fn normalize_variant_key(k: &str) -> String {
    if let Some((var, alt)) = k.split_once("-alt-") {
        if let Ok(variant) = Variant::try_from_str(var) {
            format!("{}-alt-{}", variant.as_str(), alt)
        } else {
            k.to_ascii_lowercase()
        }
    } else {
        if let Ok(variant) = Variant::try_from_str(k) {
            variant.as_str().to_string()
        } else {
            k.to_ascii_lowercase()
        }
    }
}

Since to_ascii_lowercase() is a safe fallback and is sufficient for all valid variants (which are just ASCII alphanumeric), we are sticking with it for simplicity.

[robertbastian]

Variants should work though? POSIX-alt-foo parses as a Variants and normalizes to posix-alt-foo

[sffc]

I think it's a coincidence that POSIX-alt-foo parses as a Variants. The -alt-foo could be -alt-x (too short) or -alt-abbreviated (too long).

Given these options:

1. Apply to_ascii_lowercase() to the whole string (current PR)
2. Parse the first subtag as Variant and apply to_ascii_lowercase() to the rest of the string (the suggestion above in this thread, with tweaks)
3. Parse the whole string as Variants

my preference order is: 1 ~> 2 ~>> 3

Wdyt?

[robertbastian]

it should be 2. we also need to error if it doesn't parse as a variant due to length, or because it contains non-ascii characters. you probably even want to store (Variant, String) as the key, if you want to use the Variant -> DataMarkerAttributes constructor later

[robertbastian]

.

[Manishearth]

Approach seems better, will let Robert review since it's his feedback

[robertbastian]

.