refactor: unify error constructors to object params, add validationErrors

[priosshrsth]

Summary

• Replace inconsistent positional constructor params across all error subclasses with a single uniform object { message?, cause?, details? }
• AssemblyResponseParseError narrows cause to ZodError (required) and drops the old zodError field — access via .cause
• Add validationErrors: string property using z.prettifyError() (Zod 4 built-in) for human-readable issue output
• Export all option interfaces (AssemblyErrorOptions, AssemblyRateLimitErrorOptions, AssemblyResponseParseErrorOptions) from the public index

Before / After

Before (inconsistent positional params):

new AssemblyConnectionError("custom msg", { raw: "body" })
new AssemblyRateLimitError("slow down", 30, details)      // retryAfter sandwiched in
new AssemblyResponseParseError(zodError, "custom msg")    // args reversed

After (uniform object):

new AssemblyConnectionError({ message: "custom msg", details: { raw: "body" } })
new AssemblyRateLimitError({ message: "slow down", retryAfter: 30 })
new AssemblyResponseParseError({ cause: zodError, message: "custom msg" })

// readable validation output
err.validationErrors
// ✖ Invalid input: expected string, received number
//   → at user.name
// ✖ Invalid email address
//   → at user.email

Test plan

• bun test — all 68 tests pass
• bun run type-check — no type errors
• bun run lint — no lint errors

🤖 Generated with Claude Code