[Web UI] Support text-based artifacts preview (not just inlineData/binary)

[StevenZhang233]

Is your feature request related to a problem? Please describe.

When using types.Part.from_text() to create text-based artifacts (like source code files), the ADK Web UI fails to display them with error "Failed to fetch artifact data".

The backend API returns the data correctly, but the frontend only handles inlineData format, not text format.

Describe the solution you'd like

The Web UI should support previewing text-based artifacts created with Part.from_text(). The frontend renderArtifact function should handle both:

• {"inlineData": {"mimeType": "...", "data": "..."}} (current)
• {"text": "..."} (not supported)

For text artifacts, display them as plain text or with syntax highlighting for code files.

Describe alternatives you've considered

1. Convert text to base64 and use Part.from_bytes() with inlineData - but this adds unnecessary complexity for text content
2. Ignore the UI error and only use API directly - works but poor UX

Additional context

Reproduction steps:

artifact = types.Part.from_text(text="print('hello')")
await tool_context.save_artifact(filename="main.py", artifact=artifact)

API response works fine:

curl "http://localhost:8000/.../artifacts/main.py"
# Returns: {"text": "print('hello')"}

Environment:
google-adk version: 1.22.1
Python: 3.13
OS: macOS

[StevenZhang233]

I'd like to work on this issue.

I've investigated the root cause:

The frontend renderArtifact function in main-*.js only checks for inlineData:

let {mimeType, data} = s.inlineData ?? {};
if (!a || !c) {
    this.handleArtifactFetchFailure(...)
}

But text artifacts created with types.Part.from_text() return {"text": "..."} format, not inlineData.

Proposed fix:

Add handling for text format artifacts before the inlineData check:

// Handle text-based artifacts
if (s.text) {
    this.renderTextContent(s.text, filename);
    return;
}

// Existing inlineData handling
let {mimeType, data} = s.inlineData ?? {};

For text artifacts, we can render them as:

• Plain text with <pre> tag
• Or with syntax highlighting based on file extension (e.g., .py, .js, .md)

Could you assign this to me? I'll submit a PR with the fix.