Add MeterReturnEnergy (BC)

[andig]

Replace #29788

See #29784 (comment)

TODO

• rework energy/ returned energy
• update documentation Document: api: split MeterEnergy into MeterImport and MeterExport docs#1053

[sourcery-ai]

Hey - I've found 2 issues, and left some high level feedback:

• In charger/measurement/energy.go, Export is documented as optional but Configure unconditionally dereferences cc.Export; when no export measurement is configured this will panic, so consider only creating exportG (and returning a non-nil third function) when an Export config is present.
• Similarly in meter/measurement/energy.go, Import/Export are described as optional with Energy as a legacy alias, but Configure currently assumes both importCfg and exportCfg are non-nil; making these getters conditional on the corresponding config being set (and returning nil functions otherwise) will better match the API and avoid nil dereferences for meters that only provide one direction.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In charger/measurement/energy.go, Export is documented as optional but Configure unconditionally dereferences cc.Export; when no export measurement is configured this will panic, so consider only creating exportG (and returning a non-nil third function) when an Export config is present.
- Similarly in meter/measurement/energy.go, Import/Export are described as optional with Energy as a legacy alias, but Configure currently assumes both importCfg and exportCfg are non-nil; making these getters conditional on the corresponding config being set (and returning nil functions otherwise) will better match the API and avoid nil dereferences for meters that only provide one direction.

## Individual Comments

### Comment 1
<location path="meter/measurement/energy.go" line_range="44-49" />
<code_context>
+		importCfg = cc.Energy
+	}
+
+	importG, err := importCfg.FloatGetter(ctx)
+	if err != nil {
+		return nil, nil, nil, fmt.Errorf("import: %w", err)
</code_context>
<issue_to_address>
**issue:** Nil checks are missing for optional Import/Export configs and can panic at runtime.

`Import`, `Export`, and `Energy` are documented as optional, but `Configure` always calls `FloatGetter` on `importCfg`/`exportCfg`. When both the main field and legacy alias are nil, this will panic. Please add nil checks (e.g. `if importCfg != nil { ... }`, `if exportCfg != nil { ... }`) and leave the getter nil when no config is provided, consistent with the previous `Energy` handling.
</issue_to_address>

### Comment 2
<location path="charger/measurement/energy.go" line_range="36-41" />
<code_context>
+		importCfg = cc.Energy
+	}
+
+	importG, err := importCfg.FloatGetter(ctx)
+	if err != nil {
+		return nil, nil, nil, fmt.Errorf("import: %w", err)
 	}

-	energyG, err := cc.Energy.FloatGetter(ctx)
+	exportG, err := cc.Export.FloatGetter(ctx)
 	if err != nil {
-		return nil, nil, fmt.Errorf("energy: %w", err)
</code_context>
<issue_to_address>
**issue (bug_risk):** Optional Import/Export configs are dereferenced unconditionally, which can cause nil-pointer panics.

`Import`, `Export`, and `Energy` are optional, but `Configure` unconditionally calls `FloatGetter` on `importCfg` and `cc.Export`. If a charger is configured only with `Power` or only with legacy `Energy` (no `Export`), these calls will panic on nil. Please follow the existing pattern of returning nil getters when a config is absent: guard `FloatGetter` calls with nil checks and only set up `MeterImport`/`MeterExport` when the corresponding configs are present.
</issue_to_address>

---

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[andig]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved and pushed in 5280bab. I merged master into this branch, resolved all merge conflicts, regenerated api/implement/implementations.go, and validated with targeted Go tests plus parallel validation.

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