Store description when saving a RunInput schema

[bunchesofdonald]

This adds a private _description field to RunInput and stores a provided description when saving the schema so the UI can grab it and display it in the resume modal.

Related to #11735

Example

from prefect import flow, pause_flow_run
from prefect.input import RunInput


class TreeInput(RunInput):
    kind: str


@flow
async def tree_flow():
    await pause_flow_run(
        wait_for_input=TreeInput.with_initial_data(
            description="What kind of tree do you want to plant?"
        )
    )

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• This pull request includes tests or only affects documentation.
• This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For documentation changes:

• This pull request includes redirect settings in netlify.toml for files that are removed or renamed.

For new functions or classes in the Python SDK:

• This pull request includes helpful docstrings.
• If a new Python file was added, this pull request contains a stub page in the Python SDK docs and an entry in mkdocs.yml navigation.

[netlify]

✅ Deploy Preview for prefect-docs-preview ready!

Name	Link
🔨 Latest commit	82736d2
🔍 Latest deploy log	https://app.netlify.com/sites/prefect-docs-preview/deploys/65b952f3aace0b0008c49186
😎 Deploy Preview	https://deploy-preview-11776--prefect-docs-preview.netlify.app/api-ref/prefect/input/run_input
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[abrookins]

This looks good, I'm into it. 👍