fix: HA Number/Select/Text optimistic-mode + slow-SoC slider UX

[nanomad]

Summary

HA's MQTT Number entity defaulted to optimistic mode in our discovery payload, so on restart HA treats its locally cached value as authoritative and republishes it to the command topic — the gateway then forwards it to the vehicle. That's how target SoC ends up reset to the user's last cached HA value (#375), and the same mechanism explains the analogous battery-capacity report. `_publish_select` and `_publish_text` had the same gap.

Changes

Discovery (structural fix)

• Add `"optimistic": false` to `_publish_number`, `_publish_select`, `_publish_text` payloads so HA reads state from `state_topic` instead of treating its cache as authoritative.
• Switches and locks already had this set; binary_sensor / sensor / event are read-only so don't apply.

Dispatcher (UX + correctness)

With `optimistic: false`, HA waits for `state_topic` to confirm before updating the slider. SAIC commands can take minutes when the car is asleep, so a naive `optimistic: false` would feel broken. The dispatcher now does:

1. Eager echo on receipt — publish the requested value to the state topic before calling the SAIC API. The slider updates instantly.
2. Rollback on failure — if the SAIC call (or any handler exception) fails, republish the previously captured value so the UI snaps back to the prior setting and the user sees the rejection.
3. Skip echo when payload is invalid — `echo_payload` returns None for unparseable input, suppressing both the eager echo and the rollback.

Handlers opt in via three optional methods on `CommandHandlerBase` (`echo_state_topic`, `capture_current_state`, `echo_payload`). The 6 number-entity handlers (target SoC, total battery capacity, 4 refresh periods) implement them; everything else is unchanged.

Test plan

New tests:

• `tests/test_ha_discovery_optimistic.py` — discovery payload regression: number / select / text all carry `optimistic: false` (and switches still do, as a guard).
• `tests/handlers/test_vehicle_command.py::TestNumberEntityEagerEcho`:
  • eager echo publishes the new value to the state topic
  • SAIC failure rolls back to the prior value
  • no rollback when no prior state was captured (target_soc is None)
  • invalid payload skips echo (and therefore skips rollback)
  • switches don't trigger echo

`pytest tests` → 177 passed.

Closes #375