Soc: extrapolate from charged energy when vehicle api is unavailable

[andig]

fixes #33116

When the vehicle backend becomes unavailable during charging, the estimator so far froze the last known soc, so a configured soc limit could be exceeded until the backend returned.

The estimator's fetchedSoc == nil branch now extrapolates the soc from charged energy using the already sampled state (prevSoc + energyDelta/energyPerSocStep) — the same formula as the existing interpolation branch, so the estimate stays continuous once the backend returns. The sampled state itself remains untouched and the next fetched soc always takes precedence. The estimate is clamped to 100% and kept monotonic so a mid-outage session energy reset cannot move it backwards. This implements the nil | value | prevsoc + delta case documented in core/soc/README.md.

publishSocAndRange now passes a missing soc through to the estimator instead of skipping it. A freshly created estimator without any prior sample returns 0 and does not overwrite a known soc (same-vehicle reconnect must keep the published value). As a side effect, the published soc now also advances with charged energy between regular soc polls.

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[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="core/soc/estimator.go" line_range="97-98" />
<code_context>
-		s.log.WARN.Println("missing vehicle soc- ignored by estimator")
+		// extrapolate soc from charged energy while no vehicle soc is available,
+		// never below the current estimate to stay monotonic across energy resets
+		if energyDelta := max(chargedEnergy, 0) - s.prevChargedEnergy; s.initialSoc > 0 && energyDelta >= 0 {
+			s.vehicleSoc = min(max(s.vehicleSoc, s.prevSoc+energyDelta/s.energyPerSocStep), 100)
+			s.log.DEBUG.Printf("soc extrapolated: %.2f%%", s.vehicleSoc)
+		}
</code_context>
<issue_to_address>
**issue (bug_risk):** A vehicle whose first valid fetched SOC is exactly 0% never enters the nil-SOC extrapolation branch because `initialSoc > 0` is false, so the estimator freezes at 0% throughout a backend outage despite charged energy increasing.

**Triggers:** When charging starts at 0% SOC and the vehicle API becomes unavailable before the next SOC sample.

**Suggested fix:** Track whether a valid SOC sample exists separately from the SOC value, instead of using `initialSoc > 0` as the initialization test.
</issue_to_address>

Sourcery assessment

Approval pending. 1 finding to address first.

Blocking findings: core/soc/estimator.go:98

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[github-actions]

✅ Opened backport pull request on release/0.314.4: #33149