[WIP] Add top-level _ecephys.json model

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add top-level _ecephys.json model</issue_title>
<issue_description>We currently satisfy nearly all of the sidecar files from the schema specified and described in bids-standard/bids-specification#2307

But previously, the _ecephys.json sidecar was not required; at some point, it was made so, putting more pressure on nwb2bids to support this
</issue_description>

<agent_instructions>Read the schema for that sidecar file and build a pydantic model for it similar to the others in this repo</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add top-level _ecephys.json model #330

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.