Return 500 when OPENAI_API_KEY is missing in auth middleware

[heliosran]

Motivation

• Ensure the authentication middleware stops processing and surfaces a server-side configuration error when OPENAI_API_KEY is not set.
• Keep the existing error envelope shape { error: { message: ... } } and avoid accidental fall-through to downstream handlers.

Description

• Updated openaiAuthMiddleware() so the if (!configuredKey) branch immediately returns c.json({ error: { message: "Server configuration error" } }, 500) instead of calling next().
• Ensured there is no fall-through in that branch by returning early.
• Replaced the placeholder worker tests with focused middleware tests in test/index.spec.ts that cover missing Authorization (401), invalid bearer format (401), missing OPENAI_API_KEY (500), and valid key allowing the request (200).

Testing

• Ran npx vitest run test/index.spec.ts --config vitest.local.config.mts and all middleware tests passed (4 passed).
• Ran npm run tsc which succeeded with no type errors.
• Ran npm run lint which completed successfully.
• Note: the default npm test path in this environment fails due to a missing wrangler.jsonc file, so the focused middleware tests were run with a temporary Vitest config instead.

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Codex Task