feat: add soft_5xx mode

[luthfifahlevi]

Add SOFT_5XX third-party sensor mode and improve dex error visibility

Summary

• Adds THIRD_PARTY_SENSOR_TOGGLE_SOFT_5XX, a new sensor mode between ON and SOFT: behaves like ON (hard failure — keeps retrying until the sensor eventually times out) by default, but falls back to SOFT's bypass-after-THIRD_PARTY_SENSOR_MAX_TIME behavior specifically when dex itself returns a 5xx — since a dex outage is a dex-side problem, not genuine "data not ready yet," and shouldn't silently block the pipeline forever the way real unavailability should.
• Restructures SuperExternal3rdPartyTaskSensor.poke() from nested nested conditionals into an explicit if/elif dispatch over the toggle value, with shared _hard_check/_apply_soft_grace_period helpers so the three active modes (ON, SOFT, SOFT_5XX) share logic instead of duplicating it.
• is_upstream_data_available now returns (is_available, is_5xx_error) instead of a bare bool, via a new DexSensorAPIError exception that carries the HTTP status code so callers can distinguish a confirmed dex 5xx from other failure types (network error, 4xx, bad payload).
• Both the Dex client (getCompletenessStats in ext/dex/dex.go) and the Airflow sensor (execute_third_party_sensor in __lib.py) now log a curl-equivalent of the failing request (method + URL, auth header intentionally omitted) on error, so a failure is reproducible without cross-referencing a separate debug log line.

Test plan

• go build ./..., go vet ./ext/dex/...
• python3 -m py_compile ext/scheduler/airflow/__lib.py