[Feature] Add JsonRpcResponseIndex utility for batch response correlation

[limehee]

Problem statement

Batch response handling requires repeated custom code for id correlation, missing response detection, and success/error partitioning. This creates inconsistent behavior between integrations and increases maintenance cost.

Proposed solution

Add JsonRpcResponseIndex in jsonrpc-core to provide structured access to parsed batch responses:

• index by id for fast lookup
• findById(...), requireById(...)
• successes(), errors() views
• missing-id / duplicate-id diagnostics
• configurable duplicate-id policy (for example: reject, keep-first, keep-last)

Alternatives considered

• Keep exposing only raw List<JsonRpcIncomingResponse>
• Encourage each project to write local helper functions

These alternatives preserve flexibility but continue the same duplication and behavior drift.

JSON-RPC behavior impact

No protocol behavior change. This is a consumer-side utility for parsed responses.

Additional context

• Works with existing parser/validator output (JsonRpcIncomingResponseEnvelope).
• Include deterministic behavior docs for duplicate response IDs.
• Add focused tests for empty batch, mixed success/error, and duplicate-id paths.