fix(gateway): treat a blank GATEWAY_HOST/GATEWAY_PORT as unset - #5801

Fixes #5800

Why

Trigger: while reading the gateway startup path for another change I noticed that
get_gateway_config() passes its fallbacks as os.getenv defaults, which only apply when the
variable is absent — and this function is called at import time (app.py:1074 → create_app() →
app.py:711).

Pain: docker run -e GATEWAY_PORT=, or GATEWAY_PORT=${PORT} with PORT unset, hands us "", and
int("") raises ValueError: invalid literal for int() with base 10: ''. The whole gateway process
dies before uvicorn binds, even though uvicorn is what actually picks the listening port and this
config only feeds the startup log line at app.py:348 plus the bind address create_app() uses. A
variable that is set-but-empty means "not configured" everywhere else in this app
(app/channels/dedupe_store.py:204 does int(os.environ.get("GATEWAY_WORKERS", "1") or 1), and
app/gateway/deps.py:83-86 catches the same ValueError), so the failure is also inconsistent with
the repo's own convention.

What changed

• A blank GATEWAY_HOST or GATEWAY_PORT now falls back to the declared default (0.0.0.0 /
  8001) instead of crashing the import, so an empty env var in a compose file or a shell
  expansion is treated the same as an absent one.
• GATEWAY_PORT=0 still means port 0 (ask the OS for a free port) — the fallback is applied to
  the string before int(), not to the parsed number.
• GATEWAY_ENABLE_DOCS is deliberately untouched: for that flag "" is a value a caller can
  reasonably mean as "off", and it fails closed today rather than crashing, so there is no bug to
  fix there.

Surface area

• Backend API — endpoint / SSE event / request-response shape under backend/app
• Default behavior change — changes existing behavior without the user opting in (default model, default setting, data shape)

Only the empty-value case changes: for host it goes from "" (an unusable bind address) to
0.0.0.0, and for port from a hard ValueError to 8001. Every non-empty value parses exactly as
before.

Bug fix verification

• Test path that reproduces the bug: backend/tests/test_gateway_docs_toggle.py::test_blank_host_and_port_fall_back_to_defaults
• Did it go red on main and green on this branch? yes

# tests/test_gateway_docs_toggle.py with app/gateway/config.py reverted to origin/main (f6269fb6)
1 failed, 12 passed     -> ValueError: invalid literal for int() with base 10: ''
# same file on this branch
13 passed

test_zero_port_is_not_treated_as_unset is the control that keeps the or fallback honest about
"0"; it passes in both columns, which is the point — it pins the case a naive int(x or 8001)
would get wrong.

Validation

make is not available on this machine, so I ran the commands behind make lint / make test
directly from backend/, with PYTHONPATH=.;packages/harness;packages/extension-api against a
Python 3.12 venv:

ruff check app/gateway/config.py tests/test_gateway_docs_toggle.py   -> All checks passed!
ruff format --check <same two files>                                 -> 2 files already formatted
pytest tests/test_gateway_docs_toggle.py                             -> 13 passed
pytest tests/test_gateway_lifespan_shutdown.py                       -> 14 passed   (reads get_gateway_config, via patch)

The full make test suite was not run locally; CI is the first full pass.

Notes for the reviewer

app/gateway/__init__.py re-exports get_gateway_config, so the module import in uvicorn app.gateway.app:app is the first thing that touches this — hence "at import time" rather than "at
first request".

AI assistance

Tool(s) used: Qoder agent (Claude), operating this account autonomously.

How you used it: the agent located the asymmetry, wrote the failing tests, the fix and this
description, and measured the red/green numbers above. The checkbox below is left unticked on
purpose: no human has read the diff yet, so nobody can claim that responsibility for it at the
moment of opening. The agent will amend or close the PR on request.