SerializedMember serialization emits name:null, violating the tool output schema (omit name when null)

[IvanMurzak]

Summary

SerializedMember serialization can emit "name": null, which violates the tool's own advertised output JSON Schema and gets rejected by strict MCP clients (e.g. Copilot):

MCP error -32602: Structured content does not match the tool's output schema: data/result/name must be string

Reproduced via assets-get-data on a Material (root SerializedMember.name was null). Every tool that returns a SerializedMember tree is affected (assets-get-data, object-get-data, gameobject-component-get, gameobject-find, scene-get-data, all *-modify, etc.) because they share the same $defs entry.

Root cause

• The hard-coded schema in src/Converter/Json/SerializedMemberConverter.cs (≈ lines 33-40) declares name as {"type":"string"} (no null), and name is correctly NOT in required (only typeName is, ≈ line 75).
• But SerializedMember.name is string? (src/Model/SerializedMember.cs) and is legitimately null for root/unnamed/value-only members and SerializedMember.Null(type) placeholders.
• SerializedMemberConverter.Write (≈ line 156) always writes the name key (writer.WriteString("name", value.name)), emitting JSON null when value.name == null. null is not a string → genuine schema violation. The client is correct to reject.

Fix (targeted, minimal, low-risk)

In SerializedMemberConverter.Write, omit the name key when it is null (write it only when non-null). Because name is not in required, an absent key is schema-valid; a present key is always a string. This makes it impossible to emit data that violates our own schema for name.

Do NOT widen the schema to ["string","null"] — JsonSchema.Internal.cs PostprocessFields actively strips "null" out of type unions, so that path would fight itself. Omit-when-null is the clean fix.

Acceptance criteria

• SerializedMemberConverter.Write omits the name property when value.name == null; still writes it when non-null.
• Unit test: serializing a SerializedMember with null name produces JSON with NO name key (and validates against SerializedMemberConverter.Schema); with non-null name the key is present and a string.
• Existing tests green (net8.0 + net9.0). No version bump / no release in this PR.

Out of scope (separate follow-up)

The broader systemic nullability blind spot in JsonSchema.Internal.cs (generated schemas not honoring NRT/Nullable<T>; PostprocessFields stripping "null") — tracked separately; this issue is the targeted contract fix for SerializedMember.name.

Reported from a live assets-get-data call on a Material via Copilot.