Normalize status_data.error to list[ErrorDetail]

[rapsealk]

Sub-issue of #679.

Goal

Eliminate the dual-shape status_data.error payload (single dict vs. {collection: [...]} for MultiAgentError) by always emitting a flat errors: list[ErrorDetail].

Background

Currently src/ai/backend/manager/exceptions.py:101-143 (convert_to_status_data) produces one of two incompatible shapes depending on whether the cause is a MultiAgentError. Consumers must shape-sniff at runtime — see src/ai/backend/manager/event_dispatcher/handlers/session.py:232-235:

if status_data["error"]["name"] == "MultiAgentError":
    errors = status_data["error"]["collection"]
else:
    errors = [status_data["error"]]

Scope

Backward-compatible rollout (so backend.ai-webui can migrate independently):

1. Modify convert_to_status_data to return both:
   • The legacy error: ErrorDetail field (unchanged, for backward compat)
   • A new errors: list[ErrorDetail] field (always flat — single errors are a 1-element list, MultiAgentError is the flattened collection)
2. Update the manager-internal consumer at event_dispatcher/handlers/session.py:232-235 to read errors first, fall back to shape-sniffing only if missing.
3. Add deprecation note for the legacy error field; plan removal in a later release after backend.ai-webui migrates.

Non-goals

• DB row migration of historical status_data entries (separate sub-issue, see Define a structured JSON schema for transporting kernel status data to backend.ai-webui #679 Step 4).
• Exposing the typed schema in v2 DTOs (separate sub-issue, see Define a structured JSON schema for transporting kernel status data to backend.ai-webui #679 Step 6).
• backend.ai-webui changes (separate sub-issue, see Define a structured JSON schema for transporting kernel status data to backend.ai-webui #679 Step 7).

Acceptance criteria

• convert_to_status_data always emits errors: list[ErrorDetail] in addition to legacy error.
• event_dispatcher/handlers/session.py consumes errors first.
• Tests cover: single error → 1-element list; MultiAgentError → flattened list; debug mode preserves traceback / agent_id per entry.
• Pants quality gates pass.
• PR description flags follow-up work for webui coordination.

JIRA Issue: BA-5865