Add GUILD_MEDIA channel type

[Forbidden-A]

Summary

Checklist

• I have run nox and all the pipelines have passed.
• I have made unittests according to the code I have added/modified/deleted.

Related issues

Close #2407

[Copilot]

Pull Request Overview

Adds support for the new GUILD_MEDIA channel type across the library and tests.

• Implements create_guild_media_channel in the REST client and API layers
• Introduces GuildMediaChannel model and deserialization logic
• Adds unit tests for REST call and entity factory deserialization

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/hikari/impl/test_rest.py	Added test_create_guild_media_channel to validate REST call
tests/hikari/impl/test_entity_factory.py	Added tests for deserialize_guild_media_channel and edge cases
hikari/impl/rest.py	Implemented create_guild_media_channel in the REST client
hikari/impl/entity_factory.py	Added deserialize_guild_media_channel and mapping for new type
hikari/channels.py	Defined ChannelType.GUILD_MEDIA and GuildMediaChannel
hikari/api/rest.py	Declared abstract create_guild_media_channel and updated docs
hikari/api/entity_factory.py	Declared abstract deserialize_guild_media_channel
changes/2409.feature.md	Documented the feature addition

Comments suppressed due to low confidence (1)

hikari/api/rest.py:4727

• [nitpick] This parameter refers to the channel's position within the guild, not the category. Please update the description to avoid confusion.

position
            If provided, the position of the category.