[FIX][PHP-NEXTGEN] Missing tagged union discriminator literal (#20343)

[brianferri]

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[wing328]

@brianferri thanks for the PR

can you please briefly describe the issue?

[brianferri]

@brianferri thanks for the PR

can you please briefly describe the issue?

Of course, @wing328.
For a more detailed overview: #20343

The PHP generators, when generating tagged unions dont generate all the correct discriminator mapping values.

This is because the part that should have handled this didn't exist previously in the generic model template.

The PR adds the missing part of the template for the generation of mapped models for the discriminator values in the case of a tagged union model

(This is something that other generators already do, tested with Java, python, typescript generators)

[wing328]

FYI @JulianVennen as well who's been making a lot of improvements to php-nextgen client generator.