Refactor: Extract duplicated code in job runners

[Copilot]

Eliminated code duplication across OpenAI-compatible job runners (openai, openrouter) and API key retrieval patterns in all providers.

Changes

Created src/job/utils.ts with shared utilities:

• createStreamingGenerator() - SSE streaming response handler
• transformToolsToFunctions() - ChatTool to OpenAI function format conversion
• transformUsageData() - OpenAI usage response normalization
• getApiKey() - Environment variable fallback pattern

Refactored job runners:

• openai.ts: 105 → 79 lines (-26)
• openrouter.ts: 79 → 53 lines (-26)
• voyage.ts, fal.ts: Now use shared getApiKey()

Example

Before:

// Duplicated in openai.ts and openrouter.ts
const apiKey = options?.apiKey || process.env.OPENAI_API_KEY;
const tools = input.tools?.map((tool: ChatTool) => ({
  type: "function",
  function: { name: tool.name, description: tool.description, parameters: tool.input }
}));

After:

const apiKey = getApiKey(options, "OPENAI_API_KEY");
const tools = transformToolsToFunctions(input.tools);

Net: -15 lines, ~77 lines of duplication removed.

Original prompt

Find and refactor duplicated code

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