fix: support Notion icon object payload deserialization

[brd6]

Description

• add Resource\File\Icon and Resource\Property\IconProperty so payloads with icon.type = "icon" are deserialized instead of throwing UnsupportedFileTypeException
• preserve structured icon fields (name, color) and expose a helper URL builder (getUrl(mode)) for native Notion icon rendering
• add regression and round-trip serialization tests for endpoint/resource hydration paths

Motivation and context

Notion now returns native icon payloads shaped like:
{"icon": {"type": "icon", "icon": {"name": "book", "color": "gray"}}}.
The SDK previously had no File\Icon mapping, which caused downstream sync failures during page hydration from databases()->query().

How has this been tested?

• ./vendor/bin/phpunit (108 tests, 691 assertions)
• focused regression paths:
  • tests/Endpoint/DatabasesEndpointTest.php::testQueryDatabaseWithIconObject
  • tests/Resource/PageTest.php::testPageWithIconObject
  • tests/Resource/DatabaseTest.php::testDatabaseWithIconObject
  • tests/Resource/DataSourceTest.php::testDataSourceWithIconObject
  • tests/Resource/File/IconTest.php and tests/Resource/Property/IconPropertyTest.php (including toArray() round-trip)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

PR checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.