Alan-Jowett / sonde / 23984280815

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gateway: add handler diagnostic logging and stderr capture (#649)

* gateway: add handler diagnostic logging and stderr capture

## Problem

Handler routing failures in \handle_app_data_core()\ were completely
silent — two \eturn None\ paths had zero logging, making it impossible
to diagnose why APP_DATA was being dropped. Handler stderr was piped to
\Stdio::null()\, hiding process startup errors (e.g. Python tracebacks
from missing dependencies).

## Fix

- Add WARN logs when APP_DATA is dropped because \current_program_hash\
  is None or no handler matches (with \
ode_id\, \program_hash\, and
  \handler_count\ for diagnosis).
- Capture handler stderr via a background tokio task that logs each line
  at WARN level, tagged with the handler command.
- Add \HandlerRouter::handler_count()\ for diagnostic context.
- Remove unused \oute_app_data()\ and \oute_event()\ methods (dead
  code superseded by \ind_handler_cloned()\ + inline dispatch).

## Spec updates

- **GW-0503**: Add AC4 (stderr forwarded to gateway log).
- **GW-1308**: Add AC6 (WARN on APP_DATA drop) and AC7 (stderr capture).
- **gateway-design.md**: Update logging table, error handling table, and
  handler process lifecycle section.
- **gateway-validation.md**: Add T-0506a (stderr capture) and extend
  T-1308 with steps 10–15 for the new ACs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* style: cargo fmt

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* handler: cap stderr line length and log read errors

Address review: replace unbounded \lines()\ with \ead_line()\ into
a reusable buffer, truncate at 4 KB, and log I/O errors instead of
silently stopping the drain task.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Alan Jowett <alan.jowett@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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