fix(BA-5559): handle non-dict data field in login response

[agatha197]

Resolves BA-5559

Summary

• Fix AttributeError in login_cmd.py when the server returns a string in the data field instead of a dict
• Add isinstance checks before calling .get() on the response data field
• Handles cases like "An active login session already exists" where data is a plain string

Test plan

• Run ./bai login when an active session exists — should show a clear error message instead of crashing
• Run ./bai login --force — should override existing session successfully
• Normal login flow still works as expected

[Copilot]

Pull request overview

This PR fixes a crash in the v2 CLI login flow when the server returns a non-dict data field (e.g., a plain string message), by safely extracting an error detail string before calling .get().

Changes:

• Add isinstance(..., dict) guards when reading data["data"]["details"] during login failure handling.
• Improve resilience of OTP prompting logic by handling non-dict data payloads without raising AttributeError.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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[fregataa]

Please check lint and typecheck failure

05:35:47.35 [ERROR] Completed: Typecheck using MyPy - mypy - mypy failed (exit code 1).
src/ai/backend/client/cli/v2/login_cmd.py:51: error: Name "Any" is not defined  [name-defined]
src/ai/backend/client/cli/v2/login_cmd.py:51: note: Did you forget to import it from "typing"? (Suggestion: "from typing import Any")
Found 1 error in 1 file (checked 1029 source files)