feat(schema): preserve Claude redacted thinking

[amckinley]

Summary

• add a typed Claude reasoning extension for opaque redacted thinking blocks
• preserve the extension through AgenticMessage JSON round trips
• preserve it when streaming chunks are concatenated

Why

Anthropic requires redacted thinking blocks to be passed back unchanged on subsequent turns. The AgenticMessage schema currently has no lossless representation for that opaque payload, so Claude adapters have to discard it or misuse a visible reasoning field.

A follow-up eino-ext change uses this field in the agentic Claude adapter for response conversion and request replay.

Testing

• go test ./schema/...
• adversarial cross-model review: no findings

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