fix: interpolate ${env} in help text for positional arguments

[lawrence3699]

Fixes #556.

Problem: Using ${env} in the help text of a positional argument (arg:"") panics with "undefined variable ${env}", while it works fine for flags.

// panics
var cli struct {
    Foo string `arg:"" help:"Foo (${env})" env:"FOO"`
}

Cause: interpolateValue() only populated the env variable in updatedVars when value.Flag != nil. Positional arguments have value.Flag == nil, so env was never set.

Fix: Populate env from value.Tag.Envs for positional arguments. Also merge updatedVars into vars explicitly before interpolating help, since interpolate() skips the merge when values equal missing map keys (both "").

After: ${env} resolves to the first env var name for positional args (or "" if no env tag), matching flag behavior.

All existing tests pass; added regression tests for the three cases (env set, env unset, multiple envs).

[Copilot]

Pull request overview

Fixes a construction-time panic when ${env} is used in the help text for positional arguments by ensuring the env interpolation variable is populated for non-flag values and that updated interpolation variables are reliably visible during help interpolation.

Changes:

• Populate updatedVars["env"] for positional arguments using value.Tag.Envs.
• Explicitly merge updatedVars into vars before interpolating value.Help to avoid the “missing key + empty string” merge edge case.
• Add regression tests covering positional args with env set, env unset, and multiple envs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
kong.go	Updates interpolation logic for positional-argument help to define ${env} and avoid merge-skipping edge cases.
issue556_test.go	Adds regression tests ensuring ${env} in positional help interpolates correctly across key scenarios.

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[Copilot]

In the positional-argument branch, updatedVars["env"] is derived from value.Tag.Envs without interpolating the env tag values first. For flags, env tag values are interpolated (value.Flag.Envs[i] = interpolate(...)) and then copied into value.Tag.Envs, so env:"${someVar}" works there. For positional arguments, env:"${someVar}" would currently leave ${someVar} unexpanded (both for ${env} in help and for later env lookup in Value.Reset). Consider interpolating value.Tag.Envs in the value.Flag == nil branch (similar to the flag path) before selecting the first env name.

[Copilot]

This fix works around interpolate() skipping the updatedVars merge when keys are missing and the updated value is "" by explicitly cloning vars before interpolating help. However, the same interpolate() behavior can still affect other calls in this function that pass updatedVars directly (eg env name interpolation and placeholder interpolation) whenever any of the updated vars are "" and the corresponding key is absent from vars. A more robust fix would be to adjust interpolate() to merge when a key is missing (not only when vars[key] != val), which would remove the need for special-case merges here and prevent similar edge cases elsewhere.