fix(addie): get_schema truncates large schemas at 6K chars

[bokelley]

Closes #4397

Summary

The get_schema Addie tool truncated JSON schemas at a hardcoded 6,000-char limit using a raw substring() cut, producing syntactically broken output and hiding entire oneOf branches. creative/preview-render.json (~7.7K) had its third output_format: "both" branch cut mid-sentence; creative/preview-creative-response.json (~11K) had the variant branch hidden entirely. The truncation note compounded the issue by directing the AI to the property parameter — which only works for schema.properties, not for union-type schemas.

Changes:

• Raise SCHEMA_MAX_DISPLAY_CHARS from 6K → 50K, covering all current v3 schemas except the largest union enumerations (get-adcp-capabilities-response ~75K, brand.json ~74K — documented as known remaining limitation; both reported schemas and core/format.json ~29K, core/product.json ~25K now return in full)
• Extract formatSchemaJson() as an exported pure function, making truncation logic unit-testable without HTTP mocks
• Context-aware truncation note: property-hint for schemas with top-level properties; validate_json guidance for oneOf/allOf/anyOf-only schemas (replaces the previous dead-end list_schemas redirect)
• Fix propNames derivation when property is set: truncation hint now uses displaySchema.properties so the note points to paths the agent can actually drill into, not siblings it has already passed
• Raise PRESERVE_TOOL_RESULTS threshold in token-limiter.ts from 20K → 55K to stay coherent with new 50K JSON ceiling
• Bump CODE_VERSION to 2026.05.2 per playbook (mcp/*.ts tool implementation change)
• Add 8 unit tests for formatSchemaJson including regression guards for get_schema tool truncates large schemas mid-content #4397

Known remaining limitation: Schemas exceeding 50K chars (get-adcp-capabilities-response.json ~75K, brand.json ~74K, adagents.json ~50K borderline) will still trigger the truncation path. For these, validate_json guidance in the truncation note is the correct fallback.

Non-breaking justification

Server-side Addie tool output change only. Existing callers that worked before still work; callers that were receiving truncated data now receive complete data. No protocol schemas, task definitions, or public API surfaces changed.

Pre-PR review

• code-reviewer: approved — fixed toLocaleString('en-US') locale safety, stale JSDoc. Noted truncated JSON-in-fence is pre-existing; no blockers.
• prompt-engineer (iterative review): two blockers addressed — 20K → 50K covers high-traffic schemas; union-only truncation note replaces dead-end list_schemas with actionable validate_json guidance. propNames derivation fix added for sub-property truncation.

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See #3121
for context.

Session: https://claude.ai/code/session_01Qy5CniM5du4RLEDiTadFuJ