Fix W3CBaggagePropagator to allow empty baggage values per W3C spec

[dahyvuun]

Fixes #8465

W3CBaggagePropagator drops baggage entries with empty values like key1=, but the W3C spec defines value = *baggage-octet, meaning zero characters is valid.

.NET, Go, and Rust implementations already accept empty values — Java was the only outlier.

Changes

• Added allowEmpty flag to Element.createValueElement() to permit empty string values
• Added seenWhitespace tracking to distinguish key= (valid) from key= (invalid)
• Updated tests accordingly

[linux-foundation-easycla]

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• ✅ login: dahyvuun / name: dahyvuun (8b5d88a)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.03%. Comparing base (05dfe4e) to head (7c54f00).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8468      +/-   ##
============================================
+ Coverage     91.00%   91.03%   +0.03%     
- Complexity     7817     7823       +6     
============================================
  Files           893      893              
  Lines         23721    23727       +6     
  Branches       2364     2365       +1     
============================================
+ Hits          21588    21601      +13     
+ Misses         1410     1407       -3     
+ Partials        723      719       -4     

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[jaydeluca]

i think we want to get rid of this, as right now we are accepting key1= but dropping key1= . The spec says:

Leading and trailing whitespaces (OWS) are allowed and are not considered to be a part of the value.

So i think this test needs to no longer return empty()

[jaydeluca]

this is still not addressed. You can remove this entire seenWhitespace variable and related code branches, and then the extract_value_onlySpaces test needs to be updated to:

  @Test
  void extract_value_onlySpaces() {
    W3CBaggagePropagator propagator = W3CBaggagePropagator.getInstance();

    Context result =
        propagator.extract(Context.root(), ImmutableMap.of("baggage", "key1=     "), getter);
    Baggage expectedBaggage =
        Baggage.builder().put("key1", "").build();
    assertThat(Baggage.fromContext(result)).isEqualTo(expectedBaggage);
  }