[Feature] Enforce NullAway null-safety checks across all modules from day one

[limehee]

Problem statement

The project uses nullable contracts (@Nullable / JSpecify), but null-safety is not enforced as a compile-time gate. This leaves room for NPE regressions in constructor paths, parser/validator flows, and Spring auto-configuration wiring during refactors or feature work.

Proposed solution

Integrate Error Prone + NullAway and enforce it across target modules immediately (no phased rollout).

1. Enable Error Prone + NullAway for:
   • jsonrpc-core
   • jsonrpc-spring-webmvc
   • jsonrpc-spring-boot-autoconfigure
   • optionally samples/* under the same policy
2. Configure AnnotatedPackages:
   • com.limehee.jsonrpc
3. Make NullAway violations fail the build locally and in CI (check/ci).
4. Define suppression policy:
   • allow @SuppressWarnings("NullAway") only when necessary
   • require a short justification comment
5. Document:
   • local setup/run instructions
   • false-positive handling
   • suppression rules with examples

Alternatives considered

• SpotBugs nullness checks: useful, but weaker as a compile-time enforcement gate.
• Checker Framework: stronger type system, but higher migration and maintenance cost.
• Keep current JSpecify-only approach: low friction, but no hard regression gate.

JSON-RPC behavior impact

No protocol-level behavior change. This is a build-time safety/quality enforcement improvement.

Additional context

Acceptance criteria:

• NullAway enabled across target modules and wired into CI.
• ./gradlew check fails on NullAway violations.
• Suppressions are minimal and justified.
• Relevant docs (README/docs/CONTRIBUTING) updated.

[github-actions]

⚠️ This issue has no type:* label. Please add one of the repository type labels (for example type: bug, type: feature, type: refactor).