fix(blocks): clean up file input base64 description (#13032)

### Why / What / How

Why: The Agent File Input block's `base_64` field description has a
grammar issue and misspells "across" as "accross". The same text appears
in the generated block docs, so users can see the typo in both places.

What: Cleaned up the `base_64` description for Agent File Input and
updated the matching block documentation row.

How: Reworded the source `SchemaField` description and synchronized the
corresponding docs table entry. This is copy-only and does not change
behavior or schema shape.

### Changes 🏗️

- Updated `AgentFileInputBlock.Input.base_64` copy in
`backend/blocks/io.py`.
- Updated the matching `base_64` row in
`docs/integrations/block-integrations/basic.md`.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] `poetry run lint --skip-pyright`
- [x] Smoke check verifying
`AgentFileInputBlock.Input.model_fields["base_64"].description`

<details>
  <summary>Example test plan</summary>
  
  - [ ] Create from scratch and execute an agent with at least 3 blocks
- [ ] Import an agent from file upload, and confirm it executes
correctly
  - [ ] Upload agent to marketplace
- [ ] Import an agent from marketplace and confirm it executes correctly
  - [ ] Edit an agent from monitor, and confirm it executes correctly
</details>

#### For configuration changes:

- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)

<details>
  <summary>Examples of configuration changes</summary>

  - Changing ports
  - Adding new services that need to communicate with each other
  - Secrets or environment variable changes
  - New or infrastructure changes such as databases
</details>

Co-authored-by: malu <machengyu519@gmail.com>
Co-authored-by: Nicholas Tindle <nicholas.tindle@agpt.co>
Co-authored-by: Lucas Ma <7184042+pony-maggie@users.noreply.github.com>