Backfill additionalProperties: false in OpenAI strict tool schemas - #6865

TLDR; When a caller turns on OpenAI's strict tool-calling mode, Spring AI rewrites each tool's JSON schema to meet OpenAI's strict-mode contract but it only handles half of it. It fills in the "required" list and makes optional fields nullable, yet never adds additionalProperties: false, which OpenAI also demands on every object in the schema. Tools whose schemas were generated by Spring AI happen to work because the generator adds it earlier, but tools from MCP servers or with hand-written schemas get rejected by the OpenAI API. This change makes the strict-mode rewrite fill in additionalProperties: false wherever it is missing.

Problem

OpenAiChatModel#applyStrictModeRequirements rewrites a tool's input schema when OpenAiChatOptions#strict(true) is set: it backfills required with every property key and widens previously optional properties to nullable types, recursing into $defs, nested object properties, and array items.

OpenAI's strict function-calling mode additionally requires additionalProperties: false on every object level, and rejects requests otherwise with:

Invalid schema for function '...': 'additionalProperties' is required to be supplied and to be false.

The rewrite never emits it. Schemas produced by JsonSchemaGenerator are unaffected because forbidAdditionalProperties already adds it recursively by default — which is why the gap is invisible on the @Tool/FunctionToolCallback path. Any schema from another source hits the 400 under strict mode:

• MCP tool schemas (servers rarely emit additionalProperties: false)
• hand-written ToolDefinition#inputSchema JSON
• schemas generated with SchemaOption.ALLOW_ADDITIONAL_PROPERTIES_BY_DEFAULT

See #4422