413 fix date mention creation and deserialization

[KoditkarVedant]

Description

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Fixes #413 and #414

Type of change

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• 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 not work as expected)
• This change requires a documentation update

How Has This Been Tested?

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• Test B

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Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

Src/Notion.Client/Models/Database/RichText/RichTextMention.cs:28

• Verify that removing the [JsonProperty("date")] attribute is intentional and that the JSON serialization/deserialization still maps the 'date' field correctly to the new Date type.

public Date Date { get; set; } 

Src/Notion.Client/Api/Databases/RequestParams/DatabasesCreateParameters/MentionInput.cs:20

• Ensure that the removal of the [JsonProperty("date")] attribute is deliberate so that request payloads still correctly map the date field according to the desired JSON naming conventions.

public Date Date { get; set; }