[Flight] Fix visitAsyncNode infinite recursion with circular async nodes

[jere-co]

The visitAsyncNode function used null as both an "in progress" marker and a valid cached result. When a node was revisited during its own traversal (circular reference), the function returned null as if it were a cached result, which could lead to infinite recursion.

This bug was introduced in #35005 when visited was changed from a Set to a Map for caching return values.

This change:

• Introduces an IN_PROGRESS sentinel Symbol to distinguish nodes currently being evaluated from cached results
• Returns null on cycle detection to indicate "no I/O found on this cyclic path" (not undefined, which would signal abort semantics and skip emitting I/O info for other non-cyclic branches)
• Always caches the computed result after visitAsyncNodeImpl returns, including null/undefined values, so revisits get the real computed value instead of the sentinel

This fixes RangeError: Maximum call stack size exceeded that occurs in Next.js 15.5.0+ when using database clients like Gel/EdgeDB that create circular promise chains in their async sequences.

How did you test this change?

Validated manually in Next.js apps (15.5, 16.0.5, 16.1.0-canary) using the Gel/EdgeDB repro that previously hit the stack overflow. No React unit test added; the internal module system made mocking getAsyncSequenceFromPromise impractical from the test suite. Fix verified via integration runs only.

Reproduction Repository

https://github.com/jere-co/next-debug

This repository contains:

• A minimal reproduction case triggering the bug
• Step-by-step instructions to reproduce and verify the fix
• Root cause analysis in /docs
• A temporary patch script for affected projects

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