branch-4.1: [fix](test) Stabilize Paimon external path regression case

[Gabriel39]

Proposed changes

• Replace the probabilistic requirement that both round-robin roots appear with the stable path-set contract.
• Keep an explicit non-empty assertion so an empty metadata result cannot pass vacuously.
• Document why independent writer lifecycles do not guarantee coverage of both roots.

Problem summary

Paimon randomly initializes the round-robin position for each writer lifecycle. The regression case uses independent insert statements, so a valid finite run can select only one configured root and fail even though every file uses the configured path set.

Testing

• Compiled the changed suite with the repository Groovy 4.0.19 dependency.
• Verified the path predicate for one configured root, both roots, an empty list, and an unexpected root.
• Ran git diff --check.
• Not run: the focused external Paimon regression suite requires JDK 8 and an external Paimon test environment, which are not available in the isolated worktree.

[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

Automated code review conclusion: no blocking issues found.

Opinion: approve. This is a comment-only review as required by the automated review instructions.

Critical checkpoints:

• Correctness: the patch replaces a probabilistic both-root assertion with a non-empty, all-files configured-set assertion that matches Paimon 1.4.2 writer-lifecycle semantics.
• Regression coverage: initial A/B placement, old-file retention, refreshed C/D placement, and Spark/Doris row parity remain enforced. The removed check covered upstream Paimon root distribution, not a Doris-owned cache or writer invariant.
• Compatibility, performance, and security/configuration: no regression or new exposure found in this assertion-only change.
• Convergence and deduplication: two full review rounds completed; the second normal and risk-focused reviews both returned NO_NEW_VALUABLE_FINDINGS. All earlier candidates were independently validated and dismissed with code evidence. No inline issue was accepted.
• Validation state: the live PR head/base still match the authoritative bundle, and no existing review or inline comment required deduplication. Static formatting, license, and dependency checks are green. TeamCity COMPILE is still running, and the focused external Paimon regression suite was not run in this review runner.
• User focus: no additional focus was provided; the complete PR was reviewed.