Implement getUserIdentity syscall and fix authentication context propagation in nested contexts

[faramos]

Problem Description

When using convex-test with libraries like convex-helpers that execute triggers, two critical authentication issues occur:

1. Missing getUserIdentity syscall support: convex-test throws "convexTest does not support async syscall: 1.0/getUserIdentity" when functions try to access user identity
2. Authentication context not propagated to triggers: Triggers receive contexts with _userIdentity: null instead of the mocked identity set with withIdentity()

Root Cause Analysis

Issue 1: Missing getUserIdentity syscall

The asyncSyscallImpl() function in index.ts doesn't implement the "1.0/getUserIdentity" case, causing authentication-related functions to fail.

Issue 2: Context propagation problem

In withAuth() function (lines 1773 and 1801), the context creation overwrites the original auth context:

// Line 1773 - runTransaction
const testCtx = { ...ctx, auth, ...extraCtx };

// Line 1801 - queryFromPath  
const testCtx = { ...ctx, auth };

This means when triggers execute, they don't have access to the authentication context that was set up with withIdentity().

Expected Behavior

1. Functions should be able to call ctx.auth.getUserIdentity() without errors
2. Triggers should have access to the same authentication context that was configured with withIdentity()
3. Tests should replicate real-world authentication scenarios

Proposed Solution

Fix 1: Add getUserIdentity syscall support

Add the missing case in asyncSyscallImpl():

case "1.0/getUserIdentity": {
  const auth = getCurrentAuth(); // Need to implement this
  const identity = await auth.getUserIdentity();
  return JSON.stringify(convexToJson(identity));
}

Fix 2: Preserve authentication context in nested contexts

Modify context creation to preserve original auth when it exists:

// Line 1773 - runTransaction
const testCtx = { ...ctx, auth: ctx.auth || auth, ...extraCtx };

// Line 1801 - queryFromPath  
const testCtx = { ...ctx, auth: ctx.auth || auth };

Environment

• convex-test version: 0.0.38
• convex-helpers version: 0.1.104
• convex version: 1.27.3

Impact

This affects any project using convex-test with:

• Authentication-based functions
• Trigger-based workflows (like convex-helpers)
• Complex nested function calls that require authentication context

[siraj-samsudeen]

We hit this same issue in convex-test-provider and traced the root cause to two specific locations in withAuth() (v0.0.41):

1. queryFromPath (line ~1230) — only spreads auth, doesn't override runQuery:

const testCtx = { ...ctx, auth };

Should be:

const testCtx = { ...ctx, auth, runQuery: byType.query };

2. runTransaction (line ~1208) — same issue for mutations:

const testCtx = { ...ctx, auth, ...extraCtx };

Should be:

const testCtx = { ...ctx, auth, runQuery: byType.query, runMutation: byType.mutation, ...extraCtx };

actionFromPath already does this correctly (lines ~1254-1258), so the fix is just applying the same pattern to queries and mutations.

Context: The Convex runtime provides its own ctx.runQuery/ctx.runMutation, but these bypass the test's auth-aware dispatch. Actions override them with byType.query/byType.mutation; queries and mutations don't.

Related: siraj-samsudeen/feather-testing-convex#7