fix: wrap unavailable CAA Bloks login endpoint

[subzeroid]

Summary

• Align CAA/Bloks login text input IDs with the current app flow (<6 chars>:81 / <6 chars>:82) so send_login_request no longer returns the null-payload 404 for the helper payload.
• Wrap unavailable CAA/Bloks login endpoint failures during the BadPassword fallback as TwoFactorRequired instead of leaking low-level ClientNotFoundError.
• Preserve the original exception chain and stop before Bloks 2FA verification when the CAA context cannot be obtained.
• Add regression coverage for both the updated CAA payload shape and the 404 + field_exception + Payload returned is null fallback path reported in [BUG] #2725.

Context

Refs #2725.

Fresh v434 login HAR metadata shows successful CAA login flows use username_text_input_id=<6 chars>:81 and password_text_input_id=<6 chars>:82. Live probes on this branch confirm the old helper payload returned ClientNotFoundError/404, while the updated payload returns status=ok for com.bloks.www.bloks.caa.login.async.send_login_request.

The PR still keeps the defensive wrapper because Instagram can return the same null-payload 404 for unsupported account/session states. Full automatic completion of every flagged-account CAA flow still depends on getting a real account state that returns two_step_verification_context.

Live Account Smoke

• Parsed the freshest local account file (09-06-26_02-29-01-f516aec435b10cb570b50db0ce002097.txt) without printing credentials: 500 full rows with parseable session/auth and UUIDs.
• Checked 10 fresh candidates; 2 saved-session accounts were usable.
• On usable saved sessions, account_info() and user_info_by_username_v1("instagram") passed.
• PR-specific live probe with synthetic username/fake password:
  • old helper payload reproduced the CAA null-payload 404;
  • updated helper payload returned status=ok on both default client settings and saved account settings;
  • the fallback path wraps a live CAA ClientNotFoundError as TwoFactorRequired.

Pre-Landing Review

No issues found.

Design Review

No frontend files changed; design review skipped.

Eval Results

No prompt-related files changed; evals skipped.

Plan Completion

No plan file detected.

Test plan

• Live saved-session smoke on local test accounts
• Live CAA helper route probe with synthetic username/fake password
• .venv/bin/python -m ruff check .
• .venv/bin/python -m ruff format --check .
• .venv/bin/python -m pytest tests/regression/test_auth_story.py::AuthAndStoryRegressionTestCase::test_login_bad_password_without_context_wraps_unavailable_caa_bloks_endpoint -q
• .venv/bin/python -m pytest tests/regression/test_bloks.py::BloksRegressionTestCase::test_bloks_caa_login_send_request_uses_current_payload -q
• .venv/bin/python -m pytest tests/regression/test_auth_story.py tests/regression/test_bloks.py -q
• .venv/bin/python -m pytest tests/regression -q (661 passed, 3 skipped)