feat: default datacontenttype to application/json

[Meldiron]

What

The constructor now defaults datacontenttype to application/json instead of leaving it unset. The parameter stays nullable, so datacontenttype: null still leaves the attribute unset (and omitted from toArray()).

$event = new CloudEvent(
    type: 'com.example.user.created',
    source: 'https://example.com/user-service',
    id: uniqid(),
    data: ['userId' => '123'],
);

$event->datacontenttype; // 'application/json'

Why

JSON payloads are the overwhelmingly common case, and every caller was spelling out datacontenttype: 'application/json' by hand.

Notes

• fromArray() deliberately keeps preserving the wire form — an absent datacontenttype stays absent on the parsed event (guarded by the existing testFromArrayDoesNotFabricateDatacontenttype). Per the JSON format spec, an absent datacontenttype already implies a JSON payload, so nothing needs to be fabricated when parsing.
• toArray() on a default-constructed event now includes 'datacontenttype' => 'application/json', which is a behavior change for producers relying on the attribute being omitted.

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

The PR changes the constructor’s default datacontenttype while preserving explicit-null and wire-parsing behavior.

• Defaults constructor-created events to application/json.
• Keeps datacontenttype: null available to omit the attribute.
• Documents the new default and updates its test coverage.

Confidence Score: 5/5

This PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/CloudEvents/CloudEvent.php	Changes the constructor default to application/json while ensuring fromArray() continues to preserve an absent wire attribute.
tests/CloudEvents/CloudEventTest.php	Updates default serialization expectations and tests the explicit-null behavior.
README.md	Documents the constructor default and the explicit-null escape hatch.

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