fix(config): add RetryInterval to TestValidate_ValidConfig + TestValidate_AuthTypeNone fixtures (I-001 follow-up)

Problem:
  TestValidate_ValidConfig and TestValidate_AuthTypeNone construct a
  SchedulerConfig without RetryInterval, so Validate() fails the
  'retry interval must be at least 1 second' check at config.go:1086
  with 'retry interval must be at least 1 second'. Both tests expect
  success, so they fail whenever run.

Root cause (re-derived from source, not inherited from memory):
  git log -S 'retry interval must be at least' --source --all shows
  the validation was introduced in 8665b16 (I-001, RetryFailedJobs
  scheduler wiring). git log -- internal/config/config_test.go shows
  the test file was last touched in 43e1c89, which predates 8665b16.
  I-001 added a new Validate() rule without updating the two positive
  test fixtures — a gap in I-001's verification pass.

  This is NOT C-001 fallout. The config_test.go file was untouched by
  the C-001 closure commits 0fb7d46 and d23c268. The failure surfaced
  during the full test suite run after C-001 landed because no one
  had run 'go test ./internal/config/...' since I-001.

Scope:
  - internal/config/config_test.go (2 fixtures: TestValidate_ValidConfig,
    TestValidate_AuthTypeNone).

Implementation:
  Added 'RetryInterval: 5 * time.Minute' to both SchedulerConfig
  literals. 5 minutes matches the I-001 default at config.go:818:

    RetryInterval: getEnvDuration("CERTCTL_SCHEDULER_RETRY_INTERVAL", 5*time.Minute)

  The other two TestValidate_* tests (InvalidAuthType, APIKeyAuth_
  MissingSecret) are unaffected because they expect Validate() to
  error at the auth-type check (line 1052) or auth-secret check
  (line 1057), both of which fire before the RetryInterval check at
  line 1086.

Verification:
  - go test -count=1 -run 'TestValidate_' ./internal/config/...: PASS
  - go test -short -count=1 ./...: all packages PASS
  - go vet ./...: exit 0

Residual:
  None. This is a pure test-fixture fix — production code is unchanged.

Commit:
  8665b16 (I-001) should have included this edit. Attributed here for
  traceability.

internal/config/config_test.go

@@ -328,6 +328,7 @@ func TestValidate_ValidConfig(t *testing.T) {
 			JobProcessorInterval:        30 * time.Second,
 			AgentHealthCheckInterval:    2 * time.Minute,
 			NotificationProcessInterval: 1 * time.Minute,
+			RetryInterval:               5 * time.Minute,
 		},
 	}
 	if err := cfg.Validate(); err != nil {
@@ -347,6 +348,7 @@ func TestValidate_AuthTypeNone(t *testing.T) {
 			JobProcessorInterval:        30 * time.Second,
 			AgentHealthCheckInterval:    2 * time.Minute,
 			NotificationProcessInterval: 1 * time.Minute,
+			RetryInterval:               5 * time.Minute,
 		},
 	}
 	if err := cfg.Validate(); err != nil {