Add parent_id and lock_permissions to reposition_channels

[rayakame]

Summary

• Move reposition_channels to special_endpoints without breaking/deprecating anything.
• Add parent_id and lock_permissionsfor every channel.

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

closes #523

[rayakame]

This should be the first Iteration of this feature.

Tests and docs are not done yet!
I want to clear up some things before I start working on these things.

1. Naming
   I'm not really sure about the names yet. ChannelRepositioner doesn't sound right and same for RepositionChannelHelper .
   But i'm not creative at all and wanted to ask about your guys opinions.
2. Implementation
   With the current implementation we wouldn't need to deprecate anything and there are no breaking changes. Only problem
   being that it could be pretty confusing since you can pass channels into the rest.reposition_channels and into the
   ChannelRepositioner. Normally I wouldn't say that this is important enough for a breaking change or a deprecation but we
   already have breaking changes planned for the next release so might as well do that.

Let me know about your opinions and I would be happy to implement them.

[rayakame]

I will take a look in a hour or so

[davfsa]

Only small things. Thanks for working on this!

[Copilot]

Pull Request Overview

This PR moves reposition_channels into the special_endpoints builder pattern and adds support for parent_id and lock_permissions when repositioning channels.

• Introduces ChannelRepositioner and RepositionedChannel implementations in hikari/impl/special_endpoints.py
• Changes RESTClient.reposition_channels to return a repositioner builder instead of executing immediately
• Updates API definitions, documentation, and tests to cover the new parameters

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
hikari/impl/special_endpoints.py	Adds ChannelRepositioner and RepositionedChannel classes, including parent and lock_permissions support
hikari/impl/rest.py	Modifies reposition_channels to return a builder with the new parameters
hikari/api/special_endpoints.py	Defines abstract builder interfaces for channel repositioning
hikari/api/rest.py	Updates abstract reposition_channels to return the builder and documents usage
tests/hikari/impl/test_special_endpoints.py	Adds tests for the new builder methods and parameters

Comments suppressed due to low confidence (3)

hikari/impl/special_endpoints.py:303

• The docstring references RepositionChannelHelper, but the abstract class is named RepositionedChannel. Please update the reference to match the class name.

class RepositionedChannel(special_endpoints.RepositionedChannel):

hikari/api/rest.py:5720

• The parameter docs describe a mapping from channel to position, but the signature is Mapping[int, Channel] (position → channel). Please reconcile the description with the actual key/value ordering.

        positions

hikari/api/special_endpoints.py:234

• The abstract add_reposition_channel requires lock_permissions and parent without defaults, but the implementation accepts undefined.UndefinedOr defaults. Consider making the API signature optional to match the implementation.

    def add_reposition_channel(