fix(datetime): raise specific "Missing zone offset" error in  timestamptz_to_nanos (#3505)

# Rationale for this change

  `timestamptz_to_nanos` should raise a clear "Missing zone
  offset" error when it's given a nanosecond timestamp that has no
  timezone. Today it can't: the check meant to detect a zone-less
  timestamp accidentally tests the *same* pattern as the success
  check right above it, so it never matches. The input falls
  through to the vague "Invalid timestamp with zone" error
  instead.

  The fix is one line check the zone-less pattern
  (`ISO_TIMESTAMP_NANO`) instead, mirroring 
  `timestamptz_to_micros`, which already does this correctly.
  (Introduced in #1632.)

  ## Are these changes tested?

  Yes added tests covering the error messages for both
  `timestamptz_to_nanos` and `timestamp_to_nanos`.
  
  ## Are there any user-facing changes?

  Only a clearer error message: a zone-less nanosecond timestamp
  now reports "Missing zone offset" instead of "Invalid timestamp
  with zone". No API changes.