[REQ] Add validation warning when schema default value is not in enum

[ChihweiLHBird]

Description

When a schema has default not in enum, multiple generators produce incorrect code (e.g., Python emits unquoted defaults causing NameError). This is technically valid JSON Schema but a common spec authoring mistake.

validate --recommend should warn per occurrence, e.g.:

Warnings:
    - Schema has default value 'http' not in enum [udp]

openapi-generator version

master (7.22.0-SNAPSHOT)

OpenAPI declaration file content or url

openapi: "3.0.1"
info:
  title: Test
  version: "1.0"
paths: {}
components:
  schemas:
    Config:
      type: object
      properties:
        protocol:
          type: string
          enum: [udp, tcp]
          default: http

Command line used for generation

java -jar openapi-generator-cli.jar validate -i spec.yaml --recommend

Steps to reproduce

1. Create a spec with a schema that has default not in enum (as above)
2. Run validate --recommend
3. No warning is produced about the mismatch
4. Generate code (e.g., Python) — the generated default value is incorrect

Related issues/PRs

• [BUG] Invalid default enum value with Python generator #20397 — [BUG] Invalid default enum value with Python generator
• [BUG][RUBY] Integer default enum value references constant that does not exist #17218 — [BUG][RUBY] Integer default enum value references constant that does not exist
• [BUG][KOTLIN-CLIENT] OAS 3.x: Default enum value rendered without stating the enum name as query param #21437 — [BUG][KOTLIN-CLIENT] Default enum value rendered without stating the enum name

These are all downstream symptoms of the same root cause: specs with default ∉ enum that the validator doesn't catch.

Suggest a fix/enhancement

Add a validation rule in OpenApiEvaluator.validate() that iterates all schemas via a new ModelUtils.getAllSchemasInDocument() method (covering both component and inline path schemas) and warns when schema.getDefault() ∉ schema.getEnum(). Use the per-occurrence pattern (like unused schemas) so each occurrence gets its own warning message with the specific default value and enum list.