docs: name BRIGHTDATA_API_KEY in the missing-token error and README

[anil-bd]

Why

Since 2.5.0 the client reads BRIGHTDATA_API_KEY as a fallback (TOKEN_ENV_VAR_ALT, covered by test_alt_env_var), but the missing-token error and the README only name BRIGHTDATA_API_TOKEN.

BRIGHTDATA_API_KEY is the variable the Bright Data CLI reads and the one docs.brightdata.com now uses in its samples. A user who exported it for the CLI and then hit the SDK error was told to set a different variable.

What changed

• Error message: the ValidationError for a missing token lists BRIGHTDATA_API_KEY (noting the CLI reads it too), then BRIGHTDATA_API_TOKEN.
• README: the configuration example exports BRIGHTDATA_API_KEY, and notes that BRIGHTDATA_API_TOKEN also works and wins when both are set.
• Test: test_no_credentials_error_is_actionable also asserts BRIGHTDATA_API_KEY is in the message.

Lookup order is unchanged: token argument, BRIGHTDATA_API_TOKEN, BRIGHTDATA_API_KEY, then the CLI's stored credentials.

New error text:

API token required but not found.

Provide token in one of these ways:
  1. Pass as parameter: BrightDataClient(token='your_token')
  2. Set environment variable: BRIGHTDATA_API_KEY (also read by the Bright Data CLI)
     or BRIGHTDATA_API_TOKEN
  3. Log in with the Bright Data CLI: brightdata login

Get your API token from: https://brightdata.com/cp/setting/users

Testing

• pytest tests/unit: 335 passed
• ruff check src tests: all checks passed
• black --check src tests: no changes