fix(compress-report-section): tolerate trailing characters from small-model structured output

[neoneye]

Summary

compress_premortem.md was consistently failing for plans whose premortem section is dense with multi-clause failure-mode rows (most recently 20251110_4DWW_India). The root cause is at the validator, not the model: the default LLM (openrouter-gemini-2.5-flash-lite-preview-09-2025) emits a syntactically valid JSON object then continues with extra tokens, and Pydantic's model_validate_json strict-rejects the whole payload with json_invalid ("trailing characters at line N column M"). The retry loop wastes its budget on a response whose prefix is already correct.

This PR adds a LenientJsonModel mixin in compress_report_section.py that catches that specific failure, extracts the first balanced JSON value via json.JSONDecoder().raw_decode, and validates that against the same schema. If the extracted value still fails (genuine schema problem), the original ValidationError is re-raised, so the recovery never masks structural issues.

All six per-bucket schemas (SectionSummaryOnly, NumericValuesOnly, LoadBearingAssumptionsOnly, GatesAndThresholdsOnly, RisksAndShocksOnly, MissingDataOnly) inherit the mixin.

Failure analysis

Captured against the 4DWW India case in experiments/napkin_math/docs/20260517_compress_premortem_failure.md — symptoms, failure pattern across 3 outer × 3 inner retries, root-cause hypothesis, and three escalation tiers (the validator-trim fix here is tier 1).

Verification

• 17/17 unit tests pass (4 new): trailing-object, trailing-prose, well-formed input, preserved-schema-error.
• Re-ran prepare_extract_input.py against 20251110_4DWW_India with the default LLM unchanged. compress_premortem.md and compress_premortem_raw.json were both produced on the first outer attempt, with no [premortem] Attempt N failed lines. Premortem section is now in the bundled digest.

Test plan

• pytest worker_plan_internal/parameter_extraction/tests/test_compress_report_section.py — 17/17 green
• Re-run compress on the previously-failing plan; verify premortem artifact is produced and digest contains the # Premortem section heading

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