GO-3234 Add batch object fetch endpoint

[Namasivaayam-L]

What

Adds a new endpoint POST /v1/spaces/{space_id}/objects/batch that retrieves multiple objects by their IDs in a single request. Not-found objects return null in the response array, preserving input order.

Why

• anytype-api#36 requests batch fetch by ID
• The anytype-api spec (PR anytype-api#98) defines this endpoint
• Avoids N+1 request overhead when operating on known object sets

Changes

File	Change
core/api/model/object.go	Add BatchGetObjectsRequest
core/api/service/object.go	Add BatchGetObjects method with bounded concurrency (10 parallel) + ErrFailedRetrieveBatchObject, ErrBatchTooManyIds errors
core/api/handler/object.go	Add BatchGetObjectsHandler with swag annotations
core/api/server/router.go	Register POST /spaces/:space_id/objects/batch route
core/api/docs/*	Regenerated via make openapi

Tests

• Service: 5 test cases (success, empty IDs, >100 IDs, not-found entries, order preservation)
• Router: 3 test cases (unauthorized, success with null entries, invalid body 400)
• All go test ./core/api/... passing

Post-merge tasks

• Update anytype-api spec PR speed up app closing #98 to reference this implementation
• MCP proxy will auto-generate API-get-objects-batch tool from live spec (no MCP-side changes needed)

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[Namasivaayam-L]

I have read the CLA Document and I hereby sign the CLA