[python] Fix the failing native REST snapshot tests

[sundapeng]

Purpose

apache/paimon-rust#1141 makes the Rust REST client retry GET requests on HTTP 429 and 503, as Java does. Native CI builds pypaimon-rust from paimon-rust main, so since then four tests fail on master and on every PR:

FAILED pypaimon/tests/native_hybrid_search_test.py::test_one_authoritative_rest_snapshot_for_all_routes_without_retry[error] - AssertionError: assert 6 == 1
FAILED pypaimon/tests/native_read_snapshot_test.py::test_rest_snapshot_response_is_authoritative_without_search_retry[error-vector] - AssertionError: assert 6 == 1
FAILED pypaimon/tests/native_read_snapshot_test.py::test_rest_snapshot_response_is_authoritative_without_search_retry[error-batch] - AssertionError: assert 6 == 1
FAILED pypaimon/tests/native_read_snapshot_test.py::test_rest_snapshot_response_is_authoritative_without_search_retry[error-text] - AssertionError: assert 6 == 1

They answer the snapshot request with 503 and expect one call, to check that search does not retry on its own. This PR answers with the non-retryable 500 instead, which keeps that check and leaves HTTP retries out of it.

Tests

With pypaimon-rust built from paimon-rust main (53dfa2cb5e) and the Native CI flags, native_hybrid_search_test.py and native_read_snapshot_test.py fail these four tests before the fix and pass all 87 with it.

In CI, Native CI passes; Python / Tests still fails on master's Plan.stats fallback, which #10620 fixes.

[sundapeng]

This fix is now part of #10620, which also fixes Python / Tests, so one PR can go fully green. This PR stays a draft until #10620 merges and will then be closed.

[sundapeng]

#10624 made the same change on master, so closing this.