feat(go): read union base-property dedupe facts from the IR

[fern-support]

Description

Linear ticket: Refs

PR 2 of 3 (stacked on the IR PR #17041). Makes the Go v1 model consume the discriminated-union base-property dedupe decision from the IR instead of re-deriving it. Companion to #17041 (IR facts) and the C# PR.

DRAFT — blocked. Base is will/union-base-props-ir (#17041), not main. This does nothing at runtime until (1) the new IR minor publishes and (2) go-sdk moves from irVersion: 66 → 67 — today the CLI migrates the IR down to v66 and strips these fields before the Go generator sees them. Merge order: #17041 → publish → go-sdk IR-67 upgrade → this.

Changes Made

• internal/generator/model.go: rewrote unionInheritedBasePropertyNames to read union.InheritedBaseProperties (computed once in the IR) instead of re-deriving it from the variants. Deleted everyVariantHasMatchingGetter and objectExportedProperties. Kept writeUnionInheritedBasePropertyGetters (getter emission is Go policy) and a thin local filter that keeps literal-typed inherited properties on the envelope (they render as methods, not delegatable fields).
• internal/fern/ir/types.go: hand-added the InheritedBaseProperties field + getter to the vendored UnionTypeDeclaration (stand-in for regenerating the vendored structs from the published IR).
• internal/generator/model_union_test.go: refocused the unit test on the Go side's consumption (flag gate, literal local filter, name mapping); the "which props are inherited" decision is now covered by the IR-generator tests in feat(ir): compute discriminated-union base-property dedupe facts in the IR #17041.
• Go SDK changelog entry.
• Updated README.md generator (if applicable) — N/A

Behavior note

Go previously deduped on rendered-Go-type equality (a superset of the IR's structural equality). Reading inheritedBaseProperties means Go dedupes strictly fewer cases — a few previously-suppressed fields may reappear on the envelope. This is safe (it can only keep a field, never drop a needed one → no go vet regressions) and removes the drift between the Go model and the Go dynamic snippets.

Remaining before this can merge

• feat(ir): compute discriminated-union base-property dedupe facts in the IR #17041 merges and the IR minor + @fern-fern/ir-sdk publish.
• Move go-sdk to irVersion: 67 and regenerate generators/go/internal/fern/ir/ from the published IR (replacing the hand-added struct field).
• Go dynamic-snippets consumption (DynamicTypeInstantiationMapper) — coordinate with the fix(go): stop dynamic snippets from setting deduped union base properties at the union root #16994 effort; needs @fern-api/dynamic-ir-sdk republished with inheritedBaseProperties. Not included here.
• go build ./... && go test ./... (no Go toolchain in the authoring environment) and gofmt the regenerated struct.
• Validate pnpm seed test --generator go-sdk with the flag on — expect only additive field reappearances, no removals.

Testing

• Unit test updated (model_union_test.go) — not executed locally (no Go toolchain); runs in CI once unblocked.

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