Planner: only skip short slot if more slots follow

[naltatis]

fixes #34466, refs #33936

The short slot rule (#6970, tolerance in #26814) protects the charger from short on/off cycles. Since 15m tariff slots, every plan needing less than 14m is a single trimmed slot without successor, so it was always skipped. Skipping a final slot avoids no cycle: the planner keeps proposing it until the insufficient time fallback charges at a worse price or misses the target.

• Short slots are only skipped if a later slot follows after a gap. Its remainder is then charged at the end of that later slot via the existing "continuing for remaining" path
• Short plans and the last slot of a plan now start immediately
• Fixes the insufficient time fallback being blocked as well (Frage: "plan: slot too short- ignoring" korrekt? #33936 finished 7m after the target)

🤖 Generated with Claude Code

[sourcery-ai]

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[naltatis]

\cc @iseeberg79

[andig]

Is this even an issue? If the "last" cheap slot is skipped it ends up as the very last slot in the plan, charging right before departure or before preconditioning. What does this PR do if preconditioning is active?

[naltatis]

Is this even an issue? If the "last" cheap slot is skipped it ends up as the very last slot in the plan, charging right before departure or before preconditioning. What does this PR do if preconditioning is active?

Right now a short final slot right before the goal is skipped, including a short preconditioning block. The planner then falls back to charging continuously once time runs out, which finishes late and at a higher price. The skipping was introduced to guard against on/off cycles. If no further on/off follows (last slot or part of a longer section), there is nothing to guard and we shouldn't skip.

[iseeberg79]

Yes, thinking this direction as well. Kind of special case to charge a short slot / soc range which should not hit the guard.