fix(server): project /config response to JSON-safe at the HTTP boundary

[kitlangton]

Summary

Companion to #26550 (Provider.toPublicInfo) at the /config HTTP boundary. Internal opencode state — including the live config object — may legitimately carry function/symbol/bigint/undefined values from plugin config: hooks. The product invariant is that the public HTTP API is JSON-safe data matching its typed schema, not that the internal in-memory cfg is.

This PR adds toJsonSafe and applies it in ConfigHttpApi.get only. Plugin mutation contract for config: is preserved (cfg passes through unchanged); the projection happens at the wire boundary, mirroring how Provider.toPublicInfo already projects providers in /provider and /config/providers.

Why this shape (and not in-place scrub or clone-before-hook)

• In-place scrub of cfg: would mutate live shared state. Recursing risks throwing on frozen sub-objects; non-recursing misses nested garbage. Either way it ignores the actual invariant.
• Clone before passing to plugin: breaks the documented mutation contract — fixtures and external plugins (agent-plugin.ts, etc.) register agents/providers by mutating cfg. Cloning silently discards those edits.
• Project at the HTTP boundary: matches fix(server): keep provider lists JSON-safe #26550's precedent, doesn't touch internal state, doesn't break plugin contract. Internal consumers that want raw cfg still get raw cfg; HTTP clients get the typed-schema-shaped response.

Reproducers

packages/opencode/test/plugin/mutation-repro.test.ts:

• ✅ plugin.config: plugin attaches a function to cfg, fixture asserts GET /config returns a JSON-safe body without the function. Passes after this fix.
• ⏭️ tool.definition / experimental.chat.messages.transform / experimental.chat.system.transform: skipped. Mutation IS the documented contract for these hooks. A blanket clone-and-return would break legitimate mutating plugins; a blanket in-place scrub corrupts shared Effect Schema instances in tool.parameters. Per-hook design needed — separate follow-up.

Also drops an orphan returns structured validation errors test from httpapi-sync.test.ts that no longer matches the route shape after #26550.

Test

• `bun run typecheck` — clean
• `bun run test test/plugin/mutation-repro.test.ts` — 1 pass, 3 skip, 0 fail
• `bun run test test/provider/provider.test.ts test/agent/plugin-agent-regression.test.ts` — 78/78 (no regression in plugin config-hook fixtures)

Followups (not in this PR)

• Per-hook design for the 3 trigger findings.
• Coordinate with `opencode-gemini-auth` maintainer: fix(server): keep provider lists JSON-safe #26550's clone semantics silently disabled their cost-zeroing in `auth.loader`. Recommend moving cost-norm into the `provider.models` hook, which still writes through.