feat(core): port Kimi MFJS tool-schema projection to V2

[opencode-agent]

Summary

Port the Kimi/Moonshot tool-schema compatibility projection from the V1 implementation in #37625 into the V2 provider/tool request path.

The projection prevents one incompatible built-in, custom, or MCP tool schema from causing Kimi to reject the entire model request.

Design contract

• The projected schema is availability-oriented and sent only to Kimi-selected models using the accepted provider/family/name heuristic.
• Preserve schema features accepted by Kimi and adapt only reproduced provider failures.
• Explicit types remain authoritative.
• Lossy fallbacks may widen the schema shown to the model but must never narrow valid tool inputs.
• The original tool schema remains authoritative at execution time.
• Arguments admitted by the projected schema but rejected by the original schema must become model-visible tool errors so the model can recover.

V1 reference

• Implementation and live compatibility matrix: fix(provider): normalize kimi tool schemas for mfjs #37625
• Moonshot MFJS reference: https://github.com/MoonshotAI/walle

The V1 work covers enum/type conflicts, untyped and structured enums, tuple and boolean items, typed anyOf, dangling required, local references, recursive termination, and Kimi's observed enum/union/property/size/depth limits. It preserves accepted features such as const, prefixItems, valid type arrays, nullable, composition, and conditionals.

V2 work

• Identify the V2 boundary where tool JSON Schemas are projected for provider requests without replacing execution-time schemas.
• Port the projection as a model/provider compatibility module rather than routing through V1 code.
• Apply it consistently to built-in, plugin/custom, and MCP tools.
• Preserve the original V2 tool decoder/validator through execution.
• Surface original-schema validation failures as tool results visible to the model, not fatal provider/session failures.
• Port the focused unit coverage and generic live compatibility-matrix workflow.

Acceptance criteria

• Kimi 3, Kimi 2.7 Code, and Kimi 2.6 accept the full projected compatibility matrix in V2.
• A regression test proves the projection does not narrow a representative set of valid original inputs.
• An end-to-end test forces a tool call accepted by a widened projection but rejected by the original schema, then verifies the model receives a tool error and can continue.
• Non-Kimi provider schemas remain unchanged.
• V2 contains no runtime dependency on packages/opencode.

Requested by: @rekram1-node (Aiden Cline via Slack)