fix(openapi3): preserve envelopes for variants sharing a payload type

[Timothee Guerin (timotheeguerin)]

Since 1.6.0, discriminated union variants with the same payload type reuse the first variant's envelope. This repeats its schema in oneOf and points every discriminator mapping to it, rejecting valid variants.

@discriminated(#{ discriminatorPropertyName: "tag", envelopePropertyName: "contents" })
union Limit {
  daily: string,
  weekly: string,
  monthly: string,
}

This now emits distinct LimitDaily, LimitWeekly, and LimitMonthly envelopes with matching discriminator values. Envelope caching uses the variant name within its union, preserving visibility-specific reuse.

Fixes the regression introduced by #8711.

Fixes #11979.

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commit: 352c232

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All changed packages have been documented.

• ✅ @typespec/openapi3

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@typespec/openapi3 - fix ✏️

Preserve distinct discriminated union envelopes when variants share the same payload type.

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