feat: JSON:API Atomic Operations extension support (v2.2.0)

[treere]

Summary

Implements support for the JSON:API Atomic Operations extension, allowing clients to submit a batch of add, update, and remove operations in a single request. Atomicity at the database level remains the responsibility of the calling application.

This PR targets the jsonapi1.1 branch and builds on top of the JSON:API 1.1 changes introduced in #1.

New Modules

JSONAPIPlug.AtomicPlug

A Plug.Builder-based pipeline for Atomic Operations endpoints. It:

• Enforces POST method (returns 405 otherwise).
• Delegates content-type negotiation and response content-type to the existing plugs.
• Parses and normalizes the atomic:operations request body via AtomicParams.
• Accepts api: and resources: options; resolves JSON:API type strings to resource modules at init time.

JSONAPIPlug.Document.AtomicOperation

Struct representing a single operation object within an atomic:operations document. Deserializes op, ref, href, data, and meta members; validates that op is one of add, update, or remove; enforces mutual exclusion of ref and href.

JSONAPIPlug.Document.OperationRef

Struct representing the ref member of an operation object. Requires a non-empty type and either id or lid; optionally carries a relationship member.

Changes to Existing Modules

JSONAPIPlug (main module)

• Added operations and resource_types fields to the %JSONAPIPlug{} struct.
• Added render_atomic/3: renders an atomic:results document from a list of {resource_or_nil, meta_or_nil} tuples. Returns nil when all results are empty, signalling a 204 No Content response.

JSONAPIPlug.Document

• Added operations and results fields to %Document{}.
• deserialize/1 now handles atomic:operations documents; raises InvalidDocument on mutual exclusion violations (e.g. atomic:operations + data) or an empty operations array.
• serialize/1 handles atomic:results documents, encoding results under the atomic:results key.

JSONAPIPlug.Normalizer

• denormalize/3 updated to handle atomic operation lists: returns a list of Conn.params() (one per operation) instead of a single params map.