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

[fern-support]

Description

Linear ticket: Refs

PR 3 of 3 (stacked on the IR PR #17041). Adds the opt-in C# discriminated-union base-property dedupe, reading the decision from the IR (ObjectTypeDeclaration.deferredUnionBaseProperties) instead of computing it in the generator.

Written fresh (not stacked on #16975, which will be closed), but incorporating #16975's review feedback — see below.

DRAFT — blocked. Base is will/union-base-props-ir (#17041), not main. Does not compile until (1) @fern-fern/ir-sdk publishes 67.12 with deferredUnionBaseProperties and (2) csharp moves to irVersion: 67 (declared in the versions.yml entries here; today csharp is on 66 and the CLI strips v67 fields).

Changes Made

• base/src/context/GeneratorContext.ts: new getBasePropertyWireNamesToOmitForType(typeId) that returns ir.types[typeId].deferredUnionBaseProperties wire names, gated on the flag. No computation, no reference-graph analysis, no caches.
• codegen/.../CsharpConfigSchema.ts + generation-info.ts: opt-in dedupe-union-base-properties flag (kebab-case, default off).
• model/src/generateFields.ts: extracted shared getGeneratedPropertyName helper (single source of truth for property naming).
• model/src/object/ObjectGenerator.ts: variant-leaf classes + their example initializers omit the deferred base properties.
• model/src/union/UnionGenerator.ts: generated example sets base properties on the envelope; the union JSON reader strips union-owned properties per-variant (never aggregated) before deserializing a variant.
• Unit test for getGeneratedPropertyName; versions.yml entries (csharp-sdk + csharp-model, irVersion: 67); seed fixture csharp-union-base-properties.
• Updated README.md generator (if applicable) — N/A

Learnings applied from #16975's review

• "compare value type, not just wire name" (data-corruption footgun) → solved by construction: the IR's deferredUnionBaseProperties is computed with structural type equality, so a same-named-but-differently-typed field is never dropped.
• "omission accumulates across unions / breaks standalone use" → solved by construction: the IR only marks objects used exclusively as a variant, and only props deferred by every owning union.
• "shared naming helper" → base-property naming goes through getGeneratedPropertyName.
• "use kebab-case config key" (@Swimburger) → dedupe-union-base-properties, matching every other C# config key.
• "missing versions.yml entry" → added for both csharp-sdk and csharp-model.

Verification

• Type-checked the entire change by temporarily stubbing the single deferredUnionBaseProperties access (the only line that needs the unpublished SDK field): all 17 csharp packages compile. The lone remaining error is that documented field access.

Remaining before this can merge

• feat(ir): compute discriminated-union base-property dedupe facts in the IR #17041 merges; @fern-fern/ir-sdk publishes 67.12; bump the csharp @fern-fern/ir-sdk dep and move csharp to irVersion: 67.
• C# dynamic-snippets leaf omission (the fix(go): stop dynamic snippets from setting deduped union base properties at the union root #16994-equivalent for C#) — needs @fern-api/dynamic-ir-sdk republished with deferredUnionBaseProperties; snippet tests stay disabled on the fixture until then. Not in this PR.
• Regenerate seed output (pnpm seed test --generator csharp-sdk; the fix(csharp): stop duplicating discriminated-union base properties on variant leaves #16975 baseline was only server-sent-events-openapi changing, plus this new fixture). Not committed here — no .NET toolchain in the authoring environment.

Testing

• getGeneratedPropertyName unit test added (compiles; runs in CI once unblocked).

Generated with Claude Code