refactor(core): simplify model JSON parsing

[EAGzzyCSL]

Summary

• Unify model response JSON parsing around extract -> jsonrepair -> parse -> parsed-object post-processing.
• Remove tuple-specific fallback parsing and collapse the old safe/normal parser split into parseModelResponseJsonObject.
• Route Doubao and UI-TARS JSON parsing through the shared parser while leaving bbox normalization to their post-parse coordinate logic.
• Move JSON parser unit coverage into a dedicated json.test.ts file.

Validation

• pnpm --filter @midscene/core test -- tests/unit-test/json.test.ts tests/unit-test/utils.test.ts tests/unit-test/model-adapter/doubao.test.ts tests/unit-test/model-adapter/ui-tars.test.ts
• pnpm --filter @midscene/core test -- --coverage tests/unit-test/json.test.ts

Notes

• The commit hook ran its configured lint-staged/biome checks automatically during commit.

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