Fix MCP tool output schemas to match actual serialized output

[monrax]

Description

The MCP outputSchema generated for tool output types did not match what the server actually serializes, so MCP clients that validate structuredContent against the advertised schema (the official MCP SDKs do) rejected every search_messages, aggregate_messages, and list_fields response. Three mismatches, all fixed at the schema-generator level (SchemaGeneratorProvider / victools config):

• Joda DateTime → {"type": "object"}: effective_timerange.from/.to serialize as ISO-8601 strings. A new DateTimeAsStringModule maps date/time types (Joda ReadableInstant, java.util.Date, java.time instant types, i.e. everything the object mapper writes as ISO-8601 strings) to

  {"type": "string", "format": "date-time"}

• java.lang.Object → {"type": "object", "properties": {}}: the cells of TabularResponse#datarows (List<List<Object>>) hold strings, numbers, or null. EmptyObjectAsObjectModule now skips java.lang.Object so it stays an unconstrained {} schema.
• @Nullable fields declared non-nullable: e.g. MappedFieldTypeDTO#unit serializes as null for most fields. A nullable check for javax/jakarta @Nullable now produces e.g. {"type": ["object", "null"]}.

A new round-trip test (ToolOutputSchemaComplianceTest) generates schemas and structuredContent exactly the way Tool/McpService do and validates one against the other using the MCP SDK's own JsonSchemaValidator so this class of mismatch can't silently regress.

Motivation and Context

Fixes #23980
Fixes #25314
Fixes #26402

All three issues are the same underlying bug reported against different MCP clients (Strands agents, Claude Desktop, Claude Code). With enable_output_schema turned on, the core search tools were unusable from any schema-validating client. The schema changes are strictly relaxing: every previously-valid payload remains valid, so nothing breaks for existing clients.

How Has This Been Tested?

• New ToolOutputSchemaComplianceTest (round-trip schema-vs-serialization validation for TabularResponse and ListFieldsTool.Result, plus targeted assertions for the DateTime and Object schema shapes).
• Full org.graylog.mcp.* test suite passes (McpServiceTest, McpRestResourceTest, MarkdownBuilderTest, resource provider tests).
• Live-verified against a local devserver with enable_output_schema: true: before the fix, exhaustive JSON-Schema validation of structuredContent against the advertised outputSchema produced errors on every search_messages/aggregate_messages/list_fields call; after the fix, zero validation errors across all four schema-advertising tools (including get_system_status, confirming no regression).

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

🤖 Generated with Claude Code