REST scan planning: Java iceberg-rest-fixture integration suite (Phase 6 follow-up)

[zeroshade]

Follow-up to #1494 (part of #1178, Phase 6).

The client-conformance fake added in #1494 uses hand-authored JSON fixtures. They are written to match Java's ContentFileParser conventions (typed partition values, hex-encoded binary bounds), but nothing automatically verifies they match real Java server output.

Goal

Add an integration suite, gated by RUN_INTEGRATION_TESTS=1 (matching the existing integration-test convention), that runs the REST scan-planning client against the Java apache/iceberg-rest-fixture Docker image for interop confidence.

Notes

• Mirror the existing make integration-rest / docker-compose setup.
• Confirms wire-format fidelity end-to-end (endpoint paths, JSON shapes, error types, storage credentials) against a real server, complementing the unit-level fake from test(rest): add fake server for REST scan planning #1494.

[Revanth14]

Will work on this.