Consolidate duplicate base/Enhanced API request schemas (from #10627 py-duplicate-concept)

Surfaced by the py-duplicate-concept warn rule (umbrella #10569 / #10577). Four api/schemas_*.py pairs are a base model coexisting with an Enhanced* variant of the same concept — the canonical-drift the umbrella targets, but in public API schemas, so consolidation needs migration + contract review (not a lint waiver).

Pairs (verified field analysis on Dev_new_gui)

1. ChatMessage (schemas_chat.py:461) vs EnhancedChatMessage (:520) — Enhanced is a strict superset: base fields (content,role,session_id,message_type,metadata,language) + use_ai_stack,use_knowledge_base,response_style,include_sources,thinking_mode_enabled. 11 prod usages of Enhanced.
2. SearchRequest (schemas_knowledge.py:1024) vs EnhancedSearchRequest (:1040) — Enhanced superset: base (query,limit,category) + offset,tags,tags_match_any,mode,enable_reranking,min_score,access-level. 5 usages.
3. GoalPayload (schemas_agent.py:1555) vs EnhancedGoalPayload (:1567) — same concept (submit a goal), divergent fields (base: use_phi2,user_role; enhanced: agents,coordination_mode,priority,context,use_knowledge_base,include_reasoning). 2 usages. Needs field reconciliation.
4. ChatHealthData (schemas_chat.py:217) vs EnhancedChatHealthData (:272) — base + enhanced health payload. 2 usages.

NOT a duplicate (waive in the rule, do not consolidate)

• UnifiedSearchRequest (schemas_knowledge.py:4564) — a different concept: multi-source search across facts/relations/documentation with graph expansion (top_k,doc_results,expand_relations,score_threshold,include_sources). "Unified" = unified-across-sources, a legitimate domain term, NOT a v2 of SearchRequest. Add # canonical: ignore py-duplicate-concept with this reason.

What to do (per pair, careful — public API contracts)

• Decide the canonical shape (usually the Enhanced superset becomes the one model; the base becomes an alias or is folded). Migrate all endpoint handlers + any frontend/client types. Preserve request/response field contracts (OpenAPI schema names may change — coordinate).
• Where a base is still needed for a minimal endpoint, keep it but rename the variant to a descriptive non-Enhanced name (e.g. RankedSearchRequest) so it's not a "v2 of" smell.
• Each pair can be its own PR. Once all four are resolved + UnifiedSearchRequest waived, promote py-duplicate-concept warn→block.

Note

py-duplicate-concept is intentionally a warn heuristic (#10577) — it stays warn until this lands. The other 4 umbrella rules (sync-requests, hardcoded-url, banned-suffix, adhoc-db) are now BLOCK.