fix(fraud): validate recommended_action in flag_invoice_for_review

[Jean-Regis-M]

Summary

Prevents flag_invoice_for_review from accepting invalid recommended_action strings (e.g., " hold", "HOLD") by adding explicit validation against a set of allowed actions.

Problem

The function currently accepts any string for recommended_action. Values with leading/trailing spaces (like " hold") bypass the only action‑specific check (recommended_action == "reject") and are stored unchanged in the database. This violates the documented contract and could cause silent data corruption or future logic failures.

Root Cause

There is no input validation for recommended_action. The function relies solely on an exact‑match check for "reject", which fails for any variant with extra whitespace. Any other action value (including malformed ones) is accepted without verification.

Solution

Insert a validation block immediately after confirming the invoice exists:

VALID_ACTIONS = {"hold", "reject", "escalate", "monitor"}
if recommended_action not in VALID_ACTIONS:
    raise ValueError(f"Invalid recommended_action: {recommended_action!r}. Must be one of {VALID_ACTIONS}")