🤖 perf: fix streaming content delay from ORPC schema validation

[ethanndickson]

Problem

After the ORPC migration (commit 3ee72886), streaming message content appeared delayed:

• ✅ Token count & speed updates live
• ✅ Message item (model name, time) appears immediately
• ❌ Actual message content is delayed by several seconds

messagedelays.mp4

Root Cause: Zod Union Validation Order

ORPC validates every yielded event against WorkspaceChatMessageSchema. The schema was defined as:

export const WorkspaceChatMessageSchema = z.union([
  MuxMessageSchema,                    // ← Tried FIRST for every event
  z.discriminatedUnion("type", [...]), // ← Streaming events live here
  ...
]);

The problem: z.union() tries each schema in order until one passes.

For every stream-delta event (20+ per second during streaming):

1. MuxMessageSchema is tried first → fails (wrong shape: has id/role/parts, stream-delta has type/messageId/delta)
2. Then discriminatedUnion is checked → finds matching type: "stream-delta" → passes

This failed validation attempt on MuxMessageSchema runs for every single streaming event. With rapid deltas, the overhead accumulates and causes visible delay.

Why Token Count Updated But Content Didn't

Both depend on the same event flow, but token counts are accumulated totals that remain stable once computed. Message content requires constructing new DisplayedMessage objects with accumulated text. The validation overhead delayed the entire pipeline, but cumulative metrics (tokens) appeared more responsive than the content itself.

Solution

Reorder the union to put discriminatedUnion first:

export const WorkspaceChatMessageSchema = z.union([
  z.discriminatedUnion("type", [...]),  // Most streaming events match here instantly (O(1) lookup)
  WorkspaceInitEventSchema,
  MuxMessageSchema,                      // Full messages only on stream-end/history replay
]);

Since streaming events have a type field, discriminatedUnion provides O(1) lookup - no failed validation attempts.

Also consolidated the two separate discriminatedUnion calls into one for cleaner code.

Changes

• src/common/orpc/schemas/stream.ts: Reordered union members, merged discriminated unions, added explanatory comment

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