Core: Propagate server error message in failed remote scan planning responses

[singhpk234]

Per the OpenAPI spec, FailedPlanningResult extends IcebergErrorResponse via allOf — when server-side scan planning fails, the response is an HTTP 200 with status: "failed" and a

iceberg/open-api/rest-catalog-open-api.yaml

Lines 3622 to 3633 in 8f1f483

	FailedPlanningResult:
	type: object
	description: Failed server-side planning result
	allOf:
	- $ref: '#/components/schemas/IcebergErrorResponse'
	- type: object
	required:
	- status
	properties:
	status:
	$ref: '#/components/schemas/PlanStatus'
	const: "failed"

full ErrorModel (message, type, code) in the body:

  {
    "status": "failed",
    "error": {
      "message": "I am on fire",
      "type": "IllegalStateException",
      "code": 500
    }
  }

Currently the SDK discards this error information entirely. RESTTableScan throws a generic IllegalStateException like:

Invalid planStatus: failed for planId: 6594660417062534

After this change, the server's error message is surfaced:

Received status: failed for planId: 6594660417062534. Server error: I am on fire
Changes

• Add ErrorResponse errorResponse field to FetchPlanningResultResponse and PlanTableScanResponse with an errorMessage() convenience accessor
• Parse the error object via existing ErrorResponseParser in both FetchPlanningResultResponseParser and PlanTableScanResponseParser
• Serialize the error as a spec-compliant ErrorModel (message, type, code) in toJson
• Extract ErrorResponseParser.writeErrorField() to avoid duplicating serialization logic across parsers
• Include the server error message in exceptions thrown by RESTTableScan for both planTableScan and fetchPlanningResult code paths

used ai to generate the pr