fix(agent-runtime): keep MCP schemas JSON-serializable - #877

The actual fix here is good: getMCPToolData in packages/agent-runtime/src/mcp.ts was eagerly calling convertJsonSchemaToZod at storage time, and Zod schema instances carry internal circular refs (_def, _cached, etc.) that blow up JSON.stringify when the agent state gets serialized. Storing the raw JSON Schema and deferring to ensureZodSchema at the consumption boundary (in getToolSet) is the right layer for this conversion, assuming that boundary already handles plain JSON Schema input — which the PR body claims but the diff doesn't show, since tools/prompts.ts isn't touched here. Worth double-checking there isn't some other consumer of inputSchema on MCP tool definitions that expects a Zod instance directly (e.g. calls .safeParse without going through ensureZodSchema) — if so this would just move the breakage rather than fix it.

The two new test files are enormous relative to the actual code change (555 lines of tests for a 2-line diff) and have a lot of redundant assertions (checking for _def/_cached/_type absence, multiple near-identical roundtrip checks). I'd trim these down to a handful of focused tests: one for the storage invariant (plain object, not Zod), one for the roundtrip through JSON.stringify, and one for ensureZodSchema correctly rehydrating a Zod schema for validation. As they stand they're not harmful, just more than a maintainer will want to carry forward verbatim.

Overall: correct root-cause identification, minimal core fix, in-scope package. Recommend porting the mcp.ts change with a trimmed-down test file.