fix(flags): accept rediss:// URLs for flag_fallback_cache_url

[auraseb]

💡 Motivation and Context

flag_fallback_cache_url only accepted the redis scheme. A TLS URL such as rediss://user:pass@host:6380/0?ttl=300 was rejected as an unknown scheme, a warning was logged and the flag fallback cache was disabled. Managed Redis services often require TLS, so the fallback cache could not be used there at all.

_initialize_flag_cache already rebuilds the connection URL from parsed.scheme and hands it to redis.from_url, which supports rediss://. This change lets rediss into the same branch, lists it in the error message, and adds it to the docstring examples.

💚 How did you test it?

• Added test_flag_fallback_cache_url_accepts_redis_schemes in posthog/test/test_client.py. It replaces the redis module with a mock and checks that both a redis:// and a rediss:// URL produce a RedisFlagCache with the requested TTL and that redis.from_url receives the URL with the scheme, credentials, port and db preserved and the query string removed.
• The rediss case fails on main (from_url is never called) and passes with the fix. The redis case passes on both.
• Full suite: pytest --timeout=30 passes. ruff format --check ., ruff check . and the mypy baseline check are clean.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous

• An agent found the bug, wrote the fix and the regression test, and drafted this description. Separate agent reviews reproduced the failure on main and ran the tests with the fix.
• Kept the change to the scheme check only. Rewriting the URL reconstruction (for example to preserve extra query parameters) was considered and left out to keep the diff small.

Fixes #1079

[dustinbyrne]

thank you @auraseb!