Reject non-ASCII whitespace in idn-email and strengthen email format tests

[el-psy-kongroo-d]

Follow-up to #1267, applying the review suggestions from that PR.

Changes

• Extract the non-ASCII-whitespace check into Strings.containsNonAsciiWhitespace
  and reuse it from both EmailFormat and IdnEmailFormat (instead of a private
  helper in a single class).
• idn-email: IdnEmailFormat shares EmailFormat's delegation and had the
  same bug — a leading U+00A0 was accepted. It now rejects non-ASCII whitespace
  too. Adds a new IdnEmailFormatTest (which also asserts a non-ASCII letter
  is still accepted, since idn-email allows those) and a license header to
  IdnEmailFormat, which previously had none.
• Strengthen EmailFormatTest: assert a quoted local part with an ASCII
  space ("joe bloggs"@example.com) stays valid, and generalize the negative
  case over U+00A0, U+2003 and U+3000.

Scope

Deliberately narrow: only non-ASCII whitespace is rejected. Non-ASCII
letters remain valid (important for idn-email). Zero-width format characters
such as U+200B and U+FEFF (Unicode category Cf) are out of scope and unchanged
from previous behavior.

Notes on license headers

• IdnEmailFormat had no header; the added one matches its sibling
  EmailFormat.java (Copyright (c) 2016 Network New Technologies Inc.).
• EmailFormatTest / IdnEmailFormatTest use Copyright (c) 2025 the original author or authors, matching every other test file in the format package
  (TimeFormatTest, UriFormatTest, ...). That is why the Reject non-ASCII whitespace in email format #1267 header nit was
  not switched to the main-source form — it would diverge from the test siblings.

Testing

Full test suite passes (8477 tests, 0 failures).

[el-psy-kongroo-d]

Thanks for the quick review and merge on #1267, @stevehu 🙏

This follow-up applies the suggestions from that review:

• Extracted the check into Strings.containsNonAsciiWhitespace and applied it to idn-email as well (with a test that a non-ASCII letter still validates).
• Added the quoted-local-part positive test and generalized the negative case to U+2003 / U+3000.

Two judgment calls I flagged in the description: I kept EmailFormatTest's the original author or authors header (all format test files use it, so changing only this one would diverge from its siblings), and kept the scope to whitespace only. Happy to adjust either if you'd prefer.