Fix UpdateMetadata serializer to allow null values (per-key removal)

[SanjithSambath]

Problem

inboxes.update is documented to remove a metadata key by sending it with a null value ({ metadata: { key: null } }). As of 0.5.6 the type allows this — UpdateMetadata = Record<string, MetadataValue | null> — but the serializer does not:

// before
core.serialization.record(core.serialization.string(), MetadataValue)

MetadataValue is undiscriminatedUnion([string, number, boolean]) with no null member, and record() applies it to every value with no null handling. So at runtime:

UpdateMetadata.jsonOrThrow({ foo: null }, { omitUndefined: true })
// ❌ THROWS: foo: Expected string/number/boolean. Received null.

Per-key removal compiles but throws before the request is sent — null can never reach the API. (Whole-object clear metadata: null was unaffected, since optional().json() already passes null through; only the per-key value case was broken.)

Fix

Wrap the record value schema in .nullable() so null short-circuits through serialization:

// after
core.serialization.record(core.serialization.string(), MetadataValue.nullable())

Verified:

record(string(), MetadataValue.nullable()).jsonOrThrow({ foo: null }, { omitUndefined: true })
// ✅ { "foo": null }

This also makes the runtime schema match the declared Schema<UpdateMetadata.Raw, AgentMail.inboxes.UpdateMetadata> annotation (.Raw already includes | null). tsc --project ./tsconfig.cjs.json --noEmit passes.

Root cause / follow-up

This is an upstream codegen bug: fern-typescript-node-sdk@3.60.0 honors nullable<T> in the type layer but drops .nullable() in the serialization layer for record/map values (.nullable() is emitted nowhere in the generated serialization despite the def using nullable<>).

This PR is a manual edit to a generated file as an interim fix — it will be overwritten on the next fern generate unless the generator is upgraded. Reported to the Fern team separately; the durable fix is a generator bump.

🤖 Generated with Claude Code

[cubic-dev-ai]

No issues found across 1 file

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