fix: accept signup age eligibility register flag

[subzeroid]

Summary

• mirror the aiograpi signup age eligibility fix into instagrapi
• accept Instagram's eligible_to_register response key while keeping eligible as a fallback
• add a regression test for the legacy email signup path

Verification

• RED: pytest tests/regression/test_signup.py::SignupHelperRegressionTestCase::test_signup_accepts_age_eligibility_to_register_response -q failed with AgeEligibilityError before the fix
• GREEN: same target passed after the fix
• pytest tests/regression/test_signup.py -q -> 27 passed, 3 skipped, 1 warning, 2 subtests passed
• ruff check instagrapi/mixins/signup.py tests/regression/test_signup.py -> passed
• pytest tests/regression -q -> 660 passed, 3 skipped, 33 warnings, 22 subtests passed
• live account smoke on 3 existing session accounts -> 3/3 PASS (account_info, get_timeline_feed, login_by_sessionid)

Live Notes

• I did not run destructive live account signup.
• A direct DOB eligibility endpoint probe from an existing session returned ClientForbiddenError, so the PR relies on the captured aiograpi response shape plus regression coverage for this specific key.
• Sanitized live report: /tmp/instagrapi_signup_age_account_live_smoke_sanitized.json