fix: use zod instead of zod/v4 for tool type compatibility

[mattapperson]

Summary

• Fixes type incompatibility when users define tools with plain zod imports
• The SDK was importing types from zod/v4, but users typically import from zod, causing TypeScript errors due to internal type metadata differences

Changes

• Import ZodType, ZodObject, ZodRawShape from zod instead of zod/v4 in tool-types.ts, tool.ts, and tool-executor.ts
• Keep runtime imports (toJSONSchema, ZodError) from zod/v4 as they're only available there
• Use type assertion in convertZodToJsonSchema to bridge the type gap between zod and zod/v4
• Update toJSONSchema target from openapi-3.0 to draft-7 (former isn't supported in current Zod version)
• Update tests to use draft-7 target

Error Before Fix

No overload matches this call.
Type ZodObject<{ channel: ZodString; timestamp: ZodString; name: ZodString; }, strip> is not assignable to type ZodObject<Readonly<{ [k: string]: ZodType<unknown, unknown, ZodTypeInternals<unknown, unknown>>; }>, strip>.
The types of '_zod.version.minor' are incompatible between these types.
Type 1 is not assignable to type 0.

Test plan

• Build passes (pnpm run build)
• All tests pass (npx vitest --run)
• Verified tool definition works with plain zod imports

[louisgv]

LGTM!