feat(DiscordGuild): Add missing 'theme' property

[sentry]

Description

This PR addresses issue DISCATSHARP-2GW by adding the theme property to DisCatSharp/Entities/Guild/DiscordGuild.cs.

Discord's API responses for guilds now include a theme field, which was not previously modeled in DisCatSharp. This omission caused deserialization warnings (e.g., "Found missing properties in api response for DiscordGuild") as the DiscordJson deserializer encountered an unrecognized property.

The theme property was added as a nullable string, decorated with [JsonProperty("theme", NullValueHandling = NullValueHandling.Ignore)] and [DiscordInExperiment] attributes, consistent with other experimental and unmodeled fields in the DiscordGuild class. This resolves the recurring warning events.

Fixes DISCATSHARP-2GW

Type of change

• 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?

The changes were verified by observing the absence of deserialization warnings (e.g., "Found missing properties in api response for DiscordGuild") after the theme property was added to DiscordGuild.

• Test A
• Test B

Test Configuration:

• Firmware version:
• Hardware:
• Toolchain:
• SDK:

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
• Any dependent changes have been merged and published in downstream modules

@Aiko-IT-Systems/discatsharp-reviewer

Fixes DISCATSHARP-2GW