Add __repr__ to core classes

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Summary

• Add __repr__ to BaseTask, Job, and AnticaptchaClient for better debugging and REPL experience
• BaseTask.__repr__ shows class name and public non-None attributes (inherited by all task subclasses)
• Job.__repr__ shows task_id and last known status
• AnticaptchaClient.__repr__ shows the configured host

>>> client
<AnticaptchaClient host='api.anti-captcha.com'>

>>> task
<RecaptchaV2TaskProxyless websiteURL='https://example.com' websiteKey='6Le...'>

>>> job
<Job task_id=12345 status='ready'>

Test plan

• All existing tests pass (72 passed)
• Manually verified repr output for all three classes

🤖 Generated with Claude Code