fix: set the lang browser flag from Config instead of add_argument

[bercedev]

Closes #262.

The problem

Browser.start appends the flag through Config.add_argument:

# zendriver/core/browser.py
if self.config.lang is not None:
    self.config.add_argument(f"--lang={self.config.lang}")

but add_argument rejects any argument containing "lang":

# zendriver/core/config.py
def add_argument(self, arg: str) -> None:
    if any(x in arg.lower() for x in ["headless", "data-dir", "data_dir", "no-sandbox", "no_sandbox", "lang"]):
        raise ValueError(
            '"%s" not allowed. please use one of the attributes of the Config object to set it' % arg
        )

So setting the attribute the error message points to is exactly what triggers the error. lang is unusable through every entry point:

import asyncio
import zendriver as zd

async def main():
    browser = await zd.start(lang="en-US")
    await browser.stop()

asyncio.run(main())

  File "zendriver/core/browser.py", line 362, in start
    self.config.add_argument(f"--lang={self.config.lang}")
  File "zendriver/core/config.py", line 243, in add_argument
    raise ValueError(...)
ValueError: "--lang=en-US" not allowed. please use one of the attributes of the Config object to set it

Same for Browser.create(lang=...) and Config(lang=...). The only way to get the flag through today is to bypass the attribute entirely — Config(browser_args=["--lang=en-US"]) works, because arguments passed to the constructor are not validated.

The fix

Emit the flag from Config.__call__, next to the other attribute-backed flags:

if self.user_agent:
    args.append(f"--user-agent={self.user_agent}")
if self.lang:
    args.append(f"--lang={self.lang}")
if not self.sandbox:
    args.append("--no-sandbox")

and drop the add_argument call from Browser.start. This matches how headless, user_agent and sandbox are already handled, keeps the add_argument guard intact, and needs no new API.

Tests

• tests/core/test_config.py (new): the flag is emitted when lang is set, omitted when it is not, composes with browser_args, and add_argument("--lang=…") still raises.
• tests/core/test_browser.py: a browser actually starts with lang set — this is the regression that produced the ValueError.
• tests/conftest.py: CreateBrowser accepts lang so the test above can pass it.

Verified with scripts/lint.sh (ruff + mypy clean) and the tests above on Chromium 151.

Note

Worth flagging separately: --lang does not change navigator.language, which keeps reporting the browser's installed UI language. That is Chromium behaviour rather than something this PR changes, so I left it alone — happy to open a separate issue if you would like it documented.