Expose App Router runtime errors over HMR (#98438)
Ported from #98040 by @marcoshernanz.
> [!TIP]
> Best reviewed commit by commit.
### Why?
External development tools need browser runtime-error state, not just
build errors, to display application failures. Reporting remains opt-in
so applications that do not need it avoid client serialization, HMR
transport, server formatting, buffering, and rebroadcast overhead.
### How?
Add `experimental.exposeRuntimeErrorsToHMR` for App Router development
with Webpack and Turbopack. Internal integrations can also enable the
same behavior without changing `next.config` by setting
`__NEXT_EXPOSE_RUNTIME_ERRORS_TO_HMR` to any non-empty value. When
enabled, the HMR WebSocket emits `runtimeErrors` snapshots containing:
- The current pathname and browser client/document identifiers.
- Error types, names, messages, and source-mapped stacks.
- Fatality and optional catching-boundary details (`default-global`,
`custom-global`, or `custom`).
Snapshots update as errors and navigation change. New or reconnected
observers receive the current state, and disconnecting a browser clears
its reported errors. An empty snapshot means there are no currently
reported errors; it does not guarantee application recovery.
Reporting is disabled by default. Pages Router, MCP `get_errors`, and
production behavior are unchanged.
<!-- NEXT_JS_LLM -->
Co-authored-by: Marcos Hernanz
<96699542+marcoshernanz@users.noreply.github.com>
