build: enforce NullAway and tighten nullability contracts across modules

[limehee]

Summary

Enforced NullAway checks for production source sets and aligned nullability contracts across core/autoconfigure internals.
Also applied module-level formatting updates for jsonrpc-core, jsonrpc-spring-webmvc, jsonrpc-spring-boot-autoconfigure, jsonrpc-spring-boot-starter, samples, and docs.

Key changes:

• Build quality gate:
  • Added Error Prone + NullAway in Gradle version catalog and root build.
  • Enabled NullAway for production Java compile tasks and excluded test source sets.
• Core nullability alignment:
  • Kept defensive runtime guards on public entry points.
  • Removed impossible internal null branches where flow already guarantees non-null.
  • Added/updated tests for binder/dispatcher/exception resolver behavior.
• Auto-configuration/support alignment:
  • Normalized nullable/non-null contracts in interceptor/metrics/observer/executor paths.
  • Added explicit non-null constructor guards for required collaborators.
• Formatting-only normalization:
  • core, webmvc, autoconfigure, starter, samples, and docs.

Related Issues

• Closes [Feature] Enforce NullAway null-safety checks across all modules from day one #18
• Related [Feature] Enforce NullAway null-safety checks across all modules from day one #17

Change Type

• Bug fix
• Feature
• Refactor
• Documentation
• Test
• Build/CI

JSON-RPC Impact

Describe protocol-level impact, if any:

• No protocol behavior change
• Request validation behavior changed
• Error mapping behavior changed
• Method registration/dispatch behavior changed

Details:

• This PR strengthens compile-time null-safety and code consistency; JSON-RPC protocol semantics remain unchanged.

Validation

• ./gradlew test
• ./gradlew check
• Added/updated tests for new behavior

Documentation

• Updated README (if needed)
• Added migration notes for breaking changes (if any)