build: enforce Java formatting with Spotless

[omer-cengel]

Summary

Add deterministic Java formatting with Spotless and enforce it both during Maven validation and on pull requests targeting master.

Changes

• configure Spotless Maven Plugin 3.10.2 with Eclipse 4.40
• add a repository-owned four-space Eclipse formatter profile
• remove unused imports, enforce the approved import order, forbid wildcard imports, trim trailing whitespace, and require final newlines
• bind spotless:check to Maven's validate phase while keeping spotless:apply manual
• reformat maintained production and test Java sources
• document the Java formatting policy and remediation workflow
• add a pull-request workflow that runs Spotless with Temurin Java 21 and Maven dependency caching
• pin reusable GitHub Actions to immutable commit SHAs
• use read-only permissions, disable persisted checkout credentials, cancel superseded runs, and enforce a bounded timeout

Scope and non-goals

This change affects Java formatting and its build and pull-request enforcement only. It does not change runtime behavior, generated APIs, SQL parameter or selected-column ordering, or Java text-block values.

The workflow does not run the test suite, rewrite source files, add push or scheduled triggers, introduce matrices or artifacts, configure dependency updates, or change branch-protection settings.