Forfeited by #89 revert: union defaultImpl fix + UpdateListen return-type shim + 2 regression tests

[GregHolmes]

The revert of #89 (PR #91) removed three hand-applied patches and two regression tests that are not carried forward on the re-land branch gh/sdk-gen-2026-08-11 (#92) and are not among #92's 17 reconciled patches. Filing so the next person to hit the symptom doesn't rediscover it from scratch. (Surfaced by review of #91.)

Forfeited patches

Verified on gh/sdk-gen-2026-08-11 — all three unions read defaultImpl = _UnknownValue.class, both tests absent:

1. Union default-variant fix (defaultImpl = V2Value.class) on AgentV1UpdateListenListenProvider, AgentV1SettingsAgentListenProvider, AgentV1SettingsAgentContextListenProvider.
2. AgentV1UpdateListenListen.getProvider() additive return-type shim (kept DeepgramListenProviderV2 read/write working over the union).
3. Regression guards: AgentV1UpdateListenShimTest.java, AgentSettingsProviderDefaultTest.java.

Symptom the fix addressed

The three agent listen-provider unions declare version as an optional discriminator, so a provider object with no version is a valid payload (exactly what 0.7.x emits). Fern points @JsonTypeInfo defaultImpl at _UnknownValue, whose @JsonCreator has an empty body, so the incoming value is discarded: getProvider() returns null, re-serialization emits {"provider":null} (silent provider drop on the wire), and _getUnknown()/equals() throw NullPointerException.

Calibration

The two Settings unions carry this bug in released 0.7.1 — pre-existing, not a regression. So the revert restores the released behavior; it does not make anything worse for installed users. What is forfeited is the unreleased fix.

Decision needed

Whether to re-apply the defaultImpl = V2Value fix on #92 before it merges — #92 regenerates with the same generator (4.16.0) that produces the defect. Declining is defensible (status quo for the Settings unions), but should be a deliberate decision. Owner: @GregHolmes.

[GregHolmes]

Resolved on gh/sdk-gen-2026-08-11 (#92, commit 7c5cf01):

Re-applied — the silent-data-loss fix:

• defaultImpl = V2Value on all three unions (AgentV1UpdateListenListenProvider, AgentV1SettingsAgentListenProvider, AgentV1SettingsAgentContextListenProvider), frozen in .fernignore + documented in AGENTS.md.
• AgentSettingsProviderDefaultTest re-added as the regression guard (3 tests, passing). Version-less provider payloads now deserialize as V2 instead of {"provider":null}.

Intentionally not re-applied — AgentV1UpdateListenListen.getProvider() return-type shim and AgentV1UpdateListenShimTest. The provider retype (DeepgramListenProviderV2 → AgentV1UpdateListenListenProvider union) is a deliberate breaking change for 0.8.0, documented in docs/Migrating-v0.7-to-v0.8.md and guarded by the union round-trip test in RegenTypesTest. Preserving the old return type would defeat that.

Closing — all three items are now either fixed or a documented, tested decision.