chore: add comments to the platform.proto file

[thephez]

Issue being fixed or feature implemented

The platform.proto file is not commented so tools like grpcui and protoc-gen-doc aren't as helpful as they could be.

What was done?

Added ChatGPT-derived comments for all endpoint fields

How Has This Been Tested?

Shouldn't be any functional changes. Just improves experience in grpui (tooltips show info for fields) and protoc-gen-doc (generated docs include more info).

Breaking Changes

None

Checklist:

• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated relevant unit/integration/functional/e2e tests
• [ ] I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• [ ] I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• [ ] I have assigned this pull request to a milestone

[thephez]

@shumkov I rebased this so it can be merged.