fix(signatures): build_payload raises AttributeError when targets_meta is a dict #366

Description

SignatureManager.build_payload accepts targets_meta as a single dict (it is broadcast to every execution signature), but then reads it with attribute access (target.agent_id, target.asset_id, target.asset_group_id), so passing a dict raises AttributeError. Only objects with those attributes work, e.g. the injectors' msg_data.targets_meta entries.

if isinstance(targets_meta, dict):
    targets_meta = [targets_meta] * len(execution_signatures)
...
SignatureTarget(agent=target.agent_id, ...)

Environment

1. OS: macOS 26
2. Version: main (3.261014.1)
3. Other environment details: Python 3.14

Reproducible steps

from pyoaev.signatures.models import ExecutionSignature
from pyoaev.signatures.signature_manager import SignatureManager

SignatureManager.build_payload(
    ExecutionSignature(),
    {"agent_id": "a", "asset_id": None, "asset_group_id": None},
    ["DETECTION"],
)

Expected output

The dict is used as the target metadata of every signature (or dicts are rejected with a clear error and the docstring/type hint says objects are required).

Actual output

AttributeError: 'dict' object has no attribute 'agent_id'

Additional information

No injector passes a dict today, so nothing is broken in production. Found while working on #352; #364 doesn't change this.