fix: save pasted clipboard images to temp files to support MCP tools (#14673)

[mikwiseman]

Issue for this PR

Closes #14673

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Currently, when an image is pasted from the clipboard into the prompt, it gets converted to a Base64 data URL and sent directly to the language model. While this works for multimodal models, it breaks workflows where non-vision models (like GLM-5.1) rely on external MCP tools (like zai-vision) because these tools typically expect a valid file path on disk, not a raw Base64 string.

This PR modifies the pasteAttachment function in packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx.
When an image is pasted from the clipboard (i.e. no filepath exists), it:

1. Writes the base64 content to a temporary file in os.tmpdir().
2. Uses the newly created temporary file path (file://...) as the attachment URL.

How did you verify your code works?

Compiled opencode locally (bun run build), confirmed that pasting images from clipboard now creates a temporary image file under /tmp, and verified that MCP servers can now read these files properly without throwing Base64 parsing errors.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, here are the potentially related PRs:

Related PRs found:

1. feat(tui): improve clipboard paste and drag-and-drop file handling on Linux (feat(tui): improve clipboard paste and drag-and-drop file handling on Linux #14552)

   • feat(tui): improve clipboard paste and drag-and-drop file handling on Linux #14552
   • Related because it addresses clipboard paste and file handling in the TUI, which overlaps with the current PR's focus on clipboard image attachment handling.

2. feat(tui): improve drag-drop image path handling for tool access (feat(tui): improve drag-drop image path handling for tool access #17489)

   • feat(tui): improve drag-drop image path handling for tool access #17489
   • Related because it deals with image path handling for tool access, similar to the current PR's goal of providing file paths instead of Base64 data URLs for MCP tools.

These PRs appear to address similar problems in the clipboard/drag-drop image handling space, though they may have been closed or merged already. The current PR (#21547) is specifically focused on the clipboard paste scenario with temporary file storage for MCP tool compatibility.

[github-actions]

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