refactor(core): migrate MessageV2 tool state schemas to Effect Schema

[kitlangton]

Summary

Second piecemeal slice of the session-domain Effect Schema migration (see `packages/opencode/specs/effect/schema.md`, added in #23744).

• Converts `MessageV2.ToolStatePending`, `ToolStateRunning`, `ToolStateCompleted`, `ToolStateError`, and the `ToolState` union to Effect Schema via `Schema.Struct` + derived `.zod`.
• `ToolPart` stays Zod in this slice; its `state` field now references `ToolState.zod`.
• `ToolStateCompleted.attachments` bridges to the still-Zod `FilePart` through `ZodOverride`, so array items keep emitting `$ref: #/components/schemas/FilePart`.
• Exported TS types are cast through `Types.DeepMutable` to preserve the pre-migration mutable shape used by consumer code like `compaction.ts` and `prompt.ts`.

Acceptance check

• `bun typecheck` (package + repo root) clean.
• `bun run test` → 2010 pass / 0 fail.
• `bun dev generate` output byte-identical to committed `packages/sdk/openapi.json`.
• `packages/sdk/js/src/v2/gen/types.gen.ts` unchanged after `bun script/build.ts`.

Follow-up

Listed in `specs/effect/schema.md`:

1. Part leaf schemas (`FilePart`, `TextPart`, ...) — next slice
2. `Part` discriminated union
3. `Base` / `User` / `Assistant` / `Info` / `WithParts`
4. Event payloads
5. Cursor + cleanup
6. Errors via `NamedError`-shaped bridge on `Schema.TaggedErrorClass`