Extract rule: template-simple-unless

[NullVoxPopuli]

Split from #2371.

[NullVoxPopuli-ai-agent]

Review: template-simple-unless

Compared against ember-template-lint simple-unless.js.

General Correctness

1. Missing inline unless with helper detection (default maxHelpers=1): In the original ETL rule, inline {{unless}} mustache statements with a sub-expression first param are checked via _withHelper. In the ESLint port, the GlimmerMustacheStatement handler checks node.params?.[0]?.path, but this condition is only true when the first param is a sub-expression with its own .path. This appears correct for the intended use case -- good.

2. Missing valid cases for inline unless in gjs tests: The gjs valid tests are missing several cases that the hbs tests include, such as {{unless foo bar}}, {{unless (eq foo bar) baz}}, {{unless (and isBad isAwful) "notBadAndAwful"}}, and the allowlist/maxHelpers combination tests. These should be added for completeness.

3. Missing invalid cases for inline unless in gjs tests: Several inline {{unless}} invalid cases are only tested in hbs mode but not in gjs mode (e.g., {{unless (if (or true)) 'Please no'}} with default maxHelpers). Consider adding them.

4. else unless detection using sourceCode.getText(): The isElseUnlessBlock function uses sourceCode.getText(node) and checks text.startsWith('{{else '). This is a reasonable approach for ESLint since there's no sourceForNode equivalent, but it's worth noting this may be fragile if the parser represents {{else unless}} blocks differently than expected. Worth verifying this works correctly with the actual parser output.

5. Original ETL has autofix support: The original rule supports fix mode (converting unless to if (not ...) and swapping else blocks). The ESLint port does not implement fixable. This is noted for awareness -- it's fine to skip for an initial port, but the isFixable metadata in the ETL messages suggests this was an important feature.

6. Message interpolation via withHelper messageId: The withHelper message template is '{{message}}', which works but is unusual. Typically ESLint rules use data for specific interpolation fields rather than passing the entire message string. Consider using separate messageIds for the different withHelper sub-cases (maxHelpers exceeded, allowlist violation, denylist violation).

Scope Analysis (gjs/gts)

This rule checks node.path.original === 'unless' and node.path.original === 'if'. These are Ember/Handlebars built-in control flow keywords. In gjs/gts, unless is not a JavaScript keyword and would be very unusual to shadow as a local variable. Similarly for if. No scope analysis needed -- these are structural/syntactic checks on Handlebars control flow constructs.

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