codegen: emit Default + #[serde(default)] for proto enum scalars

[prasanna-anchorage]

Problem

Bare-enum scalar fields in the generated turnkey_client crate fail to deserialize when the server response (legitimately) omits them.

Protobuf's canonical JSON mapping drops enum fields whose value is the zero / *_UNSPECIFIED variant. With the current codegen, those fields are emitted as bare enum types with neither a Default impl on the enum nor #[serde(default)] on the field, so a spec-compliant response without the key fails with:

Failed to decode response { ... } (missing field `<name>` ...)

This surfaced live for me on tvc deploy approve against a freshly-created deployment: the field was TvcDeployment.stage set to TVC_DEPLOYMENT_STAGE_UNSPECIFIED. That specific field has since been removed from the proto, but the same defect affects every other bare enum scalar in the current generated client — Outcome, InvitationStatus, TagType, Curve, AddressFormat, Effect, AccessType, Oauth2Provider, FiatOnRampProvider, CredentialType, ApiKeyCurve, and friends — any of which can fail the same way for the same reason.

Fix

Both changes live in codegen/src/transform.rs; regenerated client files in the second commit.

mutate_struct — when an enum-scalar field is rewritten from i32 to the named enum type, push #[serde(default)] for the bare (non-Vec, non-Option) case. The existing should_add_default allowlist already covers Vec<Enum> / Option<Enum>.

mutate_enum — find the variant whose explicit discriminant is 0 (the proto3 unspecified default), annotate it with #[default], and add #[derive(Default)] to the enum. Skip the flatten-style sum enums (Inner, TokenOrClaims) that already get special treatment — they aren't simple discriminated enums.

The regen diff is purely additive: #[serde(default)] on the field, #[derive(Default)] on the enum, #[default] on the zero variant. No proto changes.

Verification

• cargo build --workspace ✅
• cargo test --workspace ✅ (all existing tests pass)
• cargo clippy --all-targets --all-features -- -D warnings ✅
• cargo fmt -- --check ✅
• End-to-end: rebuilt tvc from this branch and re-ran the originally-failing tvc deploy approve --deploy-id … --operator-id … — now decodes the response and reaches the interactive approval prompt instead of erroring on a missing field.

Notes

• A codegen-template fix is the right level: the generated .rs would otherwise be wiped on the next make generate.
• make generate needed PROTOC_INCLUDE=/usr/local/include on my machine to find google/protobuf/descriptor.proto. Not a change in this PR; flagging in case it's worth documenting in the README.

🤖 Generated with Claude Code