V2 media normalization double-prefixes uppercase DATA: URLs

Description

While testing V2's OpenAI Chat request compilation, I found that an uppercase DATA: URL gets a second data-URL prefix:

Input:  DATA:image/png;base64,AAECAw==
Output: data:image/png;base64,DATA:image/png;base64,AAECAw==

Lowercase data: and raw base64 both work. Existing data URLs should be recognized regardless of scheme casing, as described in RFC 3986 §3.1.

The case-sensitive part.data.startsWith("data:") check in packages/ai/src/protocols/shared.ts sends the uppercase URL down the raw-base64 path.

I can contribute a focused fix and regression tests if this is not already being addressed.

Plugins

None; reproduced in the provider test harness.

OpenCode version

V2 source at 7a4ad68af6; Bun 1.4.2.

Steps to reproduce

In packages/ai/test/provider/openai-chat.test.ts, using the existing model, imports, and Effect test harness, compile:

const prepared = yield* compileRequest(
  LLM.request({
    model,
    messages: [
      Message.user({
        type: "media",
        mediaType: "image/png",
        data: "DATA:image/png;base64,AAECAw==",
      }),
    ],
  }),
)

The image URL in prepared.body.messages contains the double prefix shown above. Repeating with AAECAw== or data:image/png;base64,AAECAw== produces the expected URL.

This checks request encoding only: the bytes are a small test fixture, not a PNG, and no provider call is needed.

Operating System

macOS arm64.