feat(go): inject protocol-level discriminators into discriminated unions

[patrickthornton]

Description

The Go-side part of the change started in #12242. Injects the discriminator key into the union's JSON so UnmarshalJSON can just do what it normally does. If the key is already there, injection is skipped.

I.e.,

event: completion
data: { "content": "Hello!" }

becomes

event: completion
data: { "event": "completion", "content": "Hello!" }

Changes Made

Go-v2 changes.

Testing

server-side-event-examples updated to include a protocol-level case.

• Unit tests added/updated
• Manual testing completed

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