fix(bot-relay): add shutil.which step to CLI resolution and pin utf-8 decoding on delivery subprocess

Salvage hardening on top of #93601 (with #93597 covering the same core
mechanisms) for #93590:

- _hermes_cli(): after the venv-sibling check (hermes.exe on win32),
  try shutil.which('hermes') before the bare-name fallback, so
  environments with a PATH but no venv sibling resolve exactly what an
  interactive shell would. Platform test switched os.name -> sys.platform
  ('win32') per repo convention.
- tui_gateway/methods_bot_relay.py deliver: pin encoding='utf-8',
  errors='replace' on both subprocess.run sites — without them the
  child's UTF-8 output is decoded with the locale codec (cp1252/GBK on
  Windows), mangling non-ASCII replies or raising on undecodable bytes.
- Regression tests: shutil.which resolution step, bare-name fallback
  with which=None, and encoding-pin assertions in the deliver transport
  test.

Refs #93590, #93597, #93601

tui_gateway/methods_bot_relay.py

@@ -125,6 +125,8 @@ def _(rid, params: dict) -> dict:
                     local_delivery_command(resolved, tmp),
                     capture_output=True,
                     text=True,
+                    encoding="utf-8",
+                    errors="replace",
                     timeout=600,
                 )
                 if proc.returncode != 0:
@@ -147,6 +149,8 @@ def _(rid, params: dict) -> dict:
                             local_delivery_command(resolved, tmp),
                             capture_output=True,
                             text=True,
+                            encoding="utf-8",
+                            errors="replace",
                             timeout=600,
                         )
         finally: