fix(requestflag): inspect underlying type in PostParse for empty string and bool env var handling

[Adityakk9031]

Summary

In internal/requestflag/requestflag.go (Flag[T].PostParse), when handling environment variable sources, the code previously checked:

• reflect.TypeOf(f.value).Kind() == reflect.String
• reflect.TypeOf(f.value).Kind() == reflect.Bool

Because f.value is of concrete type *cliValue[T], reflect.TypeOf(f.value).Kind() always returns reflect.Pointer, never reflect.String or reflect.Bool.

This caused two bugs:

1. When an environment variable for a string flag was set to an empty string (""), val != "" evaluated to false, and the string check failed, so the flag was never populated from the environment variable.
2. When an environment variable for a boolean flag was set to "", the bool check failed, and f.Set(f.Name, "false") was never executed.

Solution

Inspect the underlying type parameter T and flag metadata using reflect.TypeFor[T]() and f.IsBoolFlag().

Tests

• Added TestFlagPostParse_EnvVars in internal/requestflag/requestflag_test.go verifying that both string flags and boolean flags correctly parse empty string environment variables during PostParse.