feat(eve): support file parts in tool model output

[BlackishGreen33]

Closes #456.

Description

Authored tools could return text or JSON to the model, but they could not return files or images. This meant a tool could create an image, while the model still had no way to inspect it.

This PR adds a content output type with text and file parts. File parts use { type: "data", data: string }; the string must contain base64-encoded data. Eve validates the fields and base64 syntax, then copies each part into a new, clean object before passing it to the AI SDK. URLs, data URLs, provider references, raw bytes, and sandbox references remain out of scope.

The original execute return value is still used for action.result, channels, and hooks. Only the value sent back to the model is normalized.

The tools documentation now includes an image example and explains the supported file source. The existing agent-tools model-output eval now returns a small PNG and checks that the model can identify it while action.result keeps the original tool result.

Fork pull requests cannot access the repository secrets required by the model-backed E2E, Vercel E2E, and TUI jobs. Those jobs now use the repository's existing fork guard pattern and report as skipped for fork pull requests. Same-repository pull requests, main pushes, and manual workflow runs still run the full checks.

How did you test your changes?

Passed with Node.js v24.15.0:

• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/harness/tools.test.ts (49 passed)
• pnpm typecheck
• pnpm lint
• pnpm exec oxfmt --check on the changed files
• pnpm check:deps
• pnpm guard:invariants
• pnpm docs:check
• pnpm test:unit
• pnpm test:integration
• pnpm build
• pnpm test:scenario
• git diff --check

I also attempted pnpm exec eve eval --strict dynamic-tools/model-output. It stopped before the model or tool call because this environment does not have AI_GATEWAY_API_KEY or VERCEL_OIDC_TOKEN. The PR is draft until that eval can run with provider credentials.

A patch changeset is included for the published eve package.

PR Checklist

• I linked an issue with prior discussion confirming this change is wanted
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant
• I added a changeset if this touches the published eve package
• DCO sign-off passes for every commit (git commit --signoff)

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