cli/token: tolerate missing token credentials when generating them

[andig]

fixes #29864

evcc token <name> aborts with cannot create vehicle type 'template': missing required accessToken before it can generate anything. cmd/token.go calls configureVehicles, which fully instantiates and validates each vehicle in RenderModeInstance; the Peugeot/PSA templates mark accessToken/refreshToken as required: true, so validation rejects the empty tokens. This is a chicken-and-egg: the command exists to produce those tokens, but config validation refuses to run without them. Regression from #24716, which switched the command to configureVehicles. (Not reproducible in go test because required-field validation is skipped under testing.Testing().)

Per the direction in the previous attempt (#30133): rather than resolving things through YAML, analyse the error and continue when it is the expected missing-credentials case.

• tokenVehicles gathers the vehicle configs (static + database) directly, without instantiating them, so the command works even while the credentials are missing
• renderTokenInstance resolves the template type via the normal RenderInstance; only when that returns a ConfigError does it retry with empty required params stubbed, so the instance type resolves. The stubbed params are exactly the empty credentials, which the token generators (PSA/ford/tronity) do not read — they perform a fresh OAuth flow

Verified against the real binary: on master evcc token car fatals with missing required +"accessToken"+`; with this change it proceeds into the PSA OAuth flow (country-code prompt) as expected.

[sourcery-ai]

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

• In renderTokenInstance, when DecodeOther or ByName fail you currently return the original validation err, which hides the real failure; consider returning or wrapping derr/terr instead so callers see the actual root cause.
• tokenVehicles appends from both conf.Vehicles and config.ConfigurationsByClass without deduplication, so a vehicle defined in both places would be processed twice; if that’s undesirable, add a simple name-based dedupe when building res.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `renderTokenInstance`, when `DecodeOther` or `ByName` fail you currently return the original validation `err`, which hides the real failure; consider returning or wrapping `derr`/`terr` instead so callers see the actual root cause.
- `tokenVehicles` appends from both `conf.Vehicles` and `config.ConfigurationsByClass` without deduplication, so a vehicle defined in both places would be processed twice; if that’s undesirable, add a simple name-based dedupe when building `res`.

## Individual Comments

### Comment 1
<location path="cmd/token.go" line_range="148-109" />
<code_context>
+		Template string
+		Other    map[string]any `mapstructure:",remain"`
+	}
+	if derr := util.DecodeOther(other, &cc); derr != nil {
+		return nil, err
+	}
+
</code_context>
<issue_to_address>
**issue (bug_risk):** Return the decode error instead of the original render error to avoid hiding the real failure cause.

If `util.DecodeOther` fails, returning the earlier `RenderInstance` error hides the actual decode failure and makes troubleshooting harder. This should return `derr` instead (optionally wrapped with additional context) so callers see the real root cause.
</issue_to_address>

### Comment 2
<location path="cmd/token.go" line_range="152-109" />
<code_context>
+		return nil, err
+	}
+
+	tmpl, terr := templates.ByName(templates.Vehicle, cc.Template)
+	if terr != nil {
+		return nil, err
+	}
+
</code_context>
<issue_to_address>
**issue (bug_risk):** Return the template lookup error instead of the original render error for clearer diagnostics.

If `templates.ByName` fails, returning the earlier `RenderInstance` error hides the actual cause of the failure. Instead, return (or wrap) `terr` so callers can see that the template lookup failed.
</issue_to_address>

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[sourcery-ai]

issue (bug_risk): Return the decode error instead of the original render error to avoid hiding the real failure cause.

If util.DecodeOther fails, returning the earlier RenderInstance error hides the actual decode failure and makes troubleshooting harder. This should return derr instead (optionally wrapped with additional context) so callers see the real root cause.

[sourcery-ai]

issue (bug_risk): Return the template lookup error instead of the original render error for clearer diagnostics.

If templates.ByName fails, returning the earlier RenderInstance error hides the actual cause of the failure. Instead, return (or wrap) terr so callers can see that the template lookup failed.