fix(config_flow): 🐛 stop options flow crashing with a 500 on default update interval

[rhammen]

🐞 Problem

Opening "Configure" (gear icon) on the integration entry gives a 500 Internal Server Error, reported across #656.

🔍 Root cause

Confirmed against a user's home-assistant.log in #656:

ERROR (MainThread) [homeassistant.helpers.http] Unable to serialize to JSON. Bad data found at
$.data_schema[1].default=0:01:00(<class 'datetime.timedelta'>, $.data_schema[1].description.suggested_value=0:01:00(<class 'datetime.timedelta'>

DEFAULT_UPDATE_INTERVAL (timedelta(seconds=60)) was reused as the fallback default for the options flow's CONF_UPDATE_INTERVAL field. That field is a SelectSelector whose valid values are the string keys of UPDATE_INTERVAL_OPTIONS ("10 seconds", "1 minute (default)", ...) — not the timedelta itself. Any config entry that had never explicitly saved options before (the common case: fresh installs, reconfigured entries, anyone who hasn't touched the update-interval dropdown) fell through to this fallback, putting a raw timedelta into the schema's default/suggested_value. The frontend serializes the schema to JSON to render the form, and timedelta isn't JSON-serializable — hence the 500.

This was introduced when the update-interval option was added (66c9fbe, 2026-06-14) and is untouched by the earlier coordinator-refresh-ordering fixes in this issue, which is why those didn't resolve it.

🔧 Fix

• Add DEFAULT_UPDATE_INTERVAL_OPTION to const.py: the UPDATE_INTERVAL_OPTIONS string key matching DEFAULT_UPDATE_INTERVAL, derived from the dict itself so the two can never drift apart.
• Use it instead of DEFAULT_UPDATE_INTERVAL in config_flow.py and schema_helper.py wherever the options-flow schema needs a default. DEFAULT_UPDATE_INTERVAL itself is untouched and still correctly used as the coordinator's own polling-interval fallback.

🧪 Testing

• New regression test: build_options_schema() with no stored interval is run through voluptuous_serialize.convert() + json.dumps() — the exact path Home Assistant's frontend uses — and must not raise. Verified this test fails with the exact reported TypeError: Object of type timedelta is not JSON serializable against the old code.
• New const-level test pinning DEFAULT_UPDATE_INTERVAL_OPTION to a valid UPDATE_INTERVAL_OPTIONS key.
• pytest -q — 746 passed
• ruff check / ruff format --check — clean
• basedpyright — 0 errors

✅ Test plan

• Confirm "Configure" (gear icon) opens without error on an entry that has never saved options before
• Confirm the update-interval dropdown still shows/saves correctly

Resolves #656

🤖 Generated with Claude Code

[github-actions]

Package	Line Rate	Health
custom_components.luxtronik2	100%	✔
Summary	100% (2881 / 2881)	✔