Lock down lenient scalar coercion in the default Jackson ObjectMapper

[OmarAlJarrah]

Problem

The default mapper only disables FAIL_ON_UNKNOWN_PROPERTIES (JacksonObjectMappers.kt:49-63). A wire string "5" silently coerces into a numeric field, masking malformed payloads.

Proposed change

Configure withCoercionConfig(...) to Fail on mismatched input shapes and disable(MapperFeature.ALLOW_COERCION_OF_SCALARS). Keep this in the Jackson adapter, not sdk-core. Do not disable the AUTO_DETECT_* features — we rely on auto-detected Kotlin data-class binding (defer any tightening to codegen, which controls its own mapper). Pre-1.0 behaviour change.

Prior art: openai-java's ObjectMappers sets per-type coercion to Fail and disables scalar coercion (Apache-2.0 — re-implement, don't copy).

Acceptance

• "5" → numeric field fails instead of coercing
• Existing data-class binding still works
• Tests

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Priority: high · Effort: small