test: add failing startup config validation tests (#52)

Encode the required fail-fast behavior as tests ahead of the
implementation: a config value that is SET but unparseable or invalid
must abort startup (defaults apply only to OMITTED keys), unknown
top-level keys and unknown metrics subkeys must abort naming the key,
a malformed config file at a standard location must abort instead of
being skipped with a warning, and state_dir must be creatable and
writable at startup. Mechanically extracts newFromSmartConfig from
config.New so the construction path is testable without fx; current
lenient behavior is unchanged, so the new enforcement tests fail.

internal/config/config.go

@@ -60,6 +60,26 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 		log.Info("no config file found, using defaults")
 	}
 
+	c, err := newFromSmartConfig(sc)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := c.ensureStateDirWritable(); err != nil {
+		return nil, err
+	}
+
+	if c.Debug {
+		params.Logger.EnableDebugLogging()
+	}
+
+	return c, nil
+}
+
+// newFromSmartConfig constructs a Config from a loaded smartconfig
+// instance and validates it. A nil sc means no config file was found,
+// in which case every option takes its default value.
+func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 	c := &Config{
 		Debug:                      getBool(sc, "debug", false),
 		MaintenanceMode:            getBool(sc, "maintenance_mode", false),
@@ -80,10 +100,6 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 		c.DBURL = fmt.Sprintf("file:%s/state.sqlite3?_journal_mode=WAL", c.StateDir)
 	}
 
-	if c.Debug {
-		params.Logger.EnableDebugLogging()
-	}
-
 	// Validate required configuration
 	if err := c.validate(); err != nil {
 		return nil, err
@@ -92,6 +108,13 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 	return c, nil
 }
 
+// ensureStateDirWritable verifies at startup that StateDir can be
+// created and written to, so a misconfigured path aborts startup
+// instead of failing later at first use.
+func (c *Config) ensureStateDirWritable() error {
+	return nil
+}
+
 // validate checks that all required configuration values are set.
 func (c *Config) validate() error {
 	if c.SigningKey == "" {