[UX] Add machine-readable MCP failure markers

[StatPan]

Parent: #60
Blocks: verification and workflow tickets that need reliable error semantics.

Goal

Add a shared response contract for machine-readable failures so LLM clients do not treat missing official data as permission to invent an answer.

Proposed Markers

• [NOT_FOUND]: official lookup completed and no matching data was found.
• [AMBIGUOUS]: multiple plausible official matches require disambiguation.
• [VERIFY_FAILED]: a user-provided claim conflicts with official data.
• [API_FAILED]: official API call failed, timed out, or returned invalid data.
• [PARTIAL]: workflow returned useful data but one or more sections failed.

Scope

• Add small helper functions or response models for marker payloads.
• Include attempted queries, source tools, suggested next queries, and an LLM instruction.
• Do not refactor every existing tool at once; wire the helper into new UX tools first.

Acceptance Criteria

• Shared marker helper or model exists.
• Unit tests cover all marker types.
• Invalid input can be represented without free-form-only strings.
• Marker payload includes enough detail for client recovery.
• No existing successful response schema is broken.