fix(mcp): strip null-valued schema types

[code-yeongyu]

Summary

Some MCP servers emit invalid JSON Schema entries with type: null. Senpi
previously copied those entries into TypeBox tool definitions unchanged, so
strict OpenAI-compatible providers rejected the request with HTTP 400 before
the model could answer.

This PR strips only JSON-null type values at the first Senpi-owned MCP schema
conversion boundary. Valid JSON Schema null declarations (type: "null" and
type: ["string", "null"]) remain unchanged, and every provider adapter now
receives the sanitized MCP tool definition.

Changes

• Omit null-valued type keys during recursive MCP schema conversion.
• Add regression coverage for root, nested property, and combiner-branch null
  types while pinning both valid null-type forms.
• Record the fork-specific MCP compatibility behavior and merge-conflict zones.

QA & Evidence

• Failing-first unit proof
  • Command: npm exec vitest -- --run packages/coding-agent/test/mcp/schema-compat.test.ts -t "strips null-valued type keywords recursively"
  • Before: failed because type: null remained at the root, nested property,
    and anyOf branch.
  • After: passed.
• Real source CLI + MCP + provider boundary
  • A local MCP stdio server advertised the malformed schema and a local
    OpenAI-compatible endpoint rejected any outbound "type":null.
  • Before: one request, one HTTP 400, null type observed, no final answer.
  • After: two requests, zero rejections, zero null types, MCP result fed into
    turn 2, final marker returned, real auth unchanged.
  • Local sanitized evidence:
    local-ignore/qa-evidence/20260804-mcp-null-schema/.
• Standard real CLI MCP regression
  • Command: node .agents/skills/senpi-qa/scripts/mock-loop.mjs --with-mcp-tool mcp_fx_tool_1 --tool-args '{"value":"ok"}' --evidence mcp-null-schema-regression
  • Result: exit 0; two model turns; fixture executed and fed back; final marker
    present; real auth unchanged.
• Automated validation
  • npm exec vitest -- --run packages/coding-agent/test/mcp/schema-compat.test.ts packages/coding-agent/test/mcp/register-call.test.ts — 16/16 passed.
  • npm run check — passed.
  • npm run build — passed across all workspaces.
  • Biome changed-file check and git diff --check — clean.

Cleanup

• RED/GREEN fake-provider ports closed.
• MCP fixture processes stopped.
• QA sandboxes removed.
• Temporary QA driver and debug journal removed.

Risks & Residuals

• The sanitizer intentionally handles only the invalid JSON null value. It does
  not alter valid "null" type semantics or other schema keywords.
• No provider-specific code changed; the fix is centralized at the MCP
  registration boundary.

Report

• Discord context: https://discord.com/channels/1452487457085063218/1490539106797621259/1534198581974663234

---

Summary by cubic

Strip invalid type: null from MCP tool input schemas during conversion to TypeBox so strict OpenAI‑compatible providers stop rejecting requests. Valid null types ("null" and ["string","null"]) are kept.

• Bug Fixes
  • Recursively omit JSON-null type keys during MCP schema conversion.
  • Preserve valid null-type declarations unchanged.
  • Add tests for root, nested property, and anyOf branch cases.
  • Record the fix in packages/coding-agent/CHANGELOG.md and mcp/changes.md.

^{Written for commit 32dd776. Summary will update on new commits.}