[fix](load): fix invalid String.format pattern in NereidsStreamLoadTask

[foxtail463]

No description provided.

[Thearas]

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?

[morrySnow]

run buildall

[morrySnow]

/review

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

No issues found in this change.

Critical checkpoint summary:

• Goal / correctness / test: The patch fixes an invalid String.format pattern in NereidsStreamLoadTask, so unsupported stream-load column expressions now raise the intended UserException instead of a formatting failure. No dedicated test was added for this error path.
• Change size / focus: Minimal and well-scoped two-line fix in the affected error messages only.
• Concurrency: Not involved.
• Lifecycle / static initialization: Not involved.
• Configuration: Not involved.
• Compatibility: No incompatible API, protocol, or storage-format change.
• Parallel code paths: I did not find the same broken format string elsewhere in FE Java sources.
• Special conditional checks: No new conditions added.
• Test coverage: Existing automated coverage for this exact failure path was not found, but the code change is straightforward and low risk.
• Observability: No additional observability needed for this fix.
• Transaction / persistence: Not involved.
• Data writes / atomicity: Not involved.
• FE/BE variable passing: Not involved.
• Performance: No impact.
• Other issues: None identified.

Applicable module checkpoints:

• FE exceptions: The method continues to surface a user-facing checked UserException; this patch only restores the intended message formatting.
• Nereids-specific review items: No rule, property-derivation, or optimizer-stage behavior is changed here.

[hello-stephen]

FE UT Coverage Report

Increment line coverage 0.00% (0/2) 🎉
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