RedactingFormatter::collect() only added a sensitive key's value to the
secret list when it was a string or Stringable. When the value was an
array, addSecret() silently dropped it, so none of its leaf strings were
collected. PsrLogMessageProcessor json-encodes array context values when
interpolating them into the message, so a secret nested that way could
already be sitting in the message in plain text by the time this formatter
ran, and the output sweep had nothing to remove it with.
Add collectSensitiveValue() to walk a sensitive key's value and collect
every string leaf, however deeply nested, and use it instead of calling
addSecret() directly.
