[fix](be) Keep Paimon JNI predicate compatibility

[Gabriel39]

Format V2 Paimon JNI reader now falls back to legacy split-level paimon_predicate when scan-level predicate is missing or empty. Added BE unit coverage for scan priority, legacy fallback, and missing predicate failure. Validation: build-support/clang-format.sh passed; ./run-be-ut.sh --run --filter=PaimonJniReaderTest* was attempted but local CMake is blocked by incomplete thirdparty/installed dependencies after missing Protobuf library.

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[Gabriel39]

run buildall

[Gabriel39]

/review

[github-actions]

Review complete. I did not find a blocking issue worth an inline review comment in this PR.

Critical checkpoint conclusions:

• Goal and correctness: the change restores format-v2 Paimon JNI predicate compatibility by preferring scan-level paimon_predicate and falling back to the legacy split-level field when the scan-level value is missing or empty. validate_scan_range() and build_scanner_params() now use the same selection rule.
• Scope: the implementation is small and localized to the v2 Paimon JNI reader plus focused BE unit coverage.
• Parallel paths: v1 Paimon JNI already prefers scan-level predicate and falls back to the split-level field; the v2 path is now aligned. Current FE planning still sets scan-level predicate/options and serialized table at scan level, and Java PaimonJniScanner consumes the resulting paimon_predicate parameter uniformly.
• Concurrency/lifecycle: no new concurrency, global state, thread lifecycle, or lock behavior was introduced.
• Configuration/session propagation: no new config item was added. Existing scan-level serialized_table, storage properties, and Paimon options propagation were checked around PaimonScanNode and FileQueryScanNode.
• Compatibility: the split-level predicate fallback is a compatibility improvement for older planning paths or rolling-upgrade scenarios that still carry the predicate in TPaimonFileDesc.
• Error handling: missing predicate still fails during scan-range validation, and the later scanner-param build asserts the same invariant after validation.
• Tests: the added BE unit tests cover scan-level precedence, missing scan-level fallback, empty scan-level fallback, and the missing-both error. The new test file is included by the BE test glob and BE UT intentionally compiles with -fno-access-control, so the protected test helper access is consistent with local test conventions.
• Validation limits: I did not run BE UT locally because this GitHub Actions checkout is missing .worktree_initialized, thirdparty/installed, and thirdparty/installed/bin/protoc; review validation was static.

User focus: no additional user-provided review focus was present.

Subagent conclusions:

• optimizer-rewrite proposed OPT-001 about protected member access in the new test. I dismissed it with evidence because doris_be_test is compiled with -fno-access-control and be/test/AGENTS.md documents that BE-UT ignores access control.
• tests-session-config found no new candidate and independently noted the same BE UT access-control configuration.
• Convergence round 1 ended with both optimizer-rewrite and tests-session-config replying NO_NEW_VALUABLE_FINDINGS for the same ledger and empty proposed inline comment set.

[hello-stephen]

BE UT Coverage Report

Increment line coverage 100.00% (14/14) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	56.53% (23198/41037)
Line Coverage	40.08% (225148/561725)
Region Coverage	36.03% (177743/493314)
Branch Coverage	37.09% (79021/213045)

[Gabriel39]

run performance

[Gabriel39]

run external