feat: support both api_key and apiKey for CUA agent API keys

[t-anjan]

why

• Python docs and examples use api_key (snake_case), e.g. at https://docs.stagehand.dev/v2/basics/agent:

  agent = stagehand.agent(
      model="claude-sonnet-4-20250514",
      instructions="You are a helpful assistant that can use a web browser.",
      options={"api_key": os.getenv("ANTHROPIC_API_KEY")},
  )
  await agent.execute("apply for a job at Browserbase")

• At https://www.browserbase.com/blog/evaluating-browser-agents, the uvx create-browser-app --template gemini-cua template also uses api_key.
• The CUA implementations were only reading apiKey from config.options, so running the generated Gemini example could error with:

  GEMINI_API_KEY environment variable not set and not provided in config.
  

  even when options={"api_key": ...} was provided.
• This is unnecessarily confusing and inconsistent between docs, templates, and runtime.

what changed

• Anthropic CUA

  • API key resolution order:
    1. config.options["api_key"]
    2. config.options["apiKey"]
    3. ANTHROPIC_API_KEY env var
  • Safely handle config / config.options being None.

• OpenAI CUA

  • API key resolution order:
    1. config.options["api_key"]
    2. config.options["apiKey"]
    3. OPENAI_API_KEY env var
  • Safely handle config / config.options being None.

• Google CUA (Gemini)

  • API key resolution order:
    1. config.options["api_key"]
    2. config.options["apiKey"]
    3. GEMINI_API_KEY env var
    4. Raise ValueError if still missing (same behavior as before).
  • Switched env var name and error messaging to GEMINI_API_KEY, matching the Gemini usage and the generated template.

test plan

• Generate and run the Gemini example:

  • uvx create-browser-app --template gemini-cua
  • Ensure it passes options={"api_key": os.getenv("GEMINI_API_KEY")}.
  • Run python main.py and verify it no longer fails with a missing GEMINI_API_KEY error when api_key is set.

• Smoke-test Anthropic and OpenAI CUA:

  • For each:
    • options={"api_key": ...}
    • options={"apiKey": ...}
    • Only the respective *_API_KEY env var set, with options omitted.
  • Confirm authentication works and no AttributeError is raised when config / options is absent.