fix(octopus): filter saving-session events by target region in Octopus Direct

[chalfontchubby]

Summary

Fixes #4612. Predbat's own "Octopus Energy Direct" GraphQL client (the direct-to-Octopus connection method, separate from BottleCapDave's HA integration) never requested or filtered on region eligibility for saving sessions, so it would attempt to auto-join events the account's own region doesn't qualify for. Some saving sessions are only valid for specific NESO grid regions - Octopus's API still lists them as "available" to every account regardless of region - so joining one triggers OE-1308: Account's region is outside of the target regions for this event.

This is the same root cause BottleCapDave hit and fixed in their integration back in v18.3.0 (BottlecapDave/HomeAssistant-OctopusEnergy#1737) - their fix never reached Predbat's Direct client since it's a fully separate, independent code path.

• Added targetRegion { regionId } to the saving-session events query and signedUpMeterPoint { regionId } to the account query (both confirmed against Octopus's live GraphQL schema via introspection, not guessed).
• get_saving_session_data() now skips any available event whose targetRegion is non-empty and doesn't include the account's own region.
• An unknown account region (signedUpMeterPoint missing/null) excludes region-restricted events rather than assuming eligibility - a wasted/rejected join attempt is exactly the failure mode this is meant to avoid. Nationwide events (no targetRegion at all) are never affected either way.

Test plan

• ./run_all --test octopus_url - two new scenarios in the existing get_saving_session_data coverage: region-mismatch excluded / matching-region and nationwide kept, and the unknown-account-region edge case (region-restricted events excluded, nationwide kept)
• ./run_all --quick passes
• ./run_pre_commit passes

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