feat: Restore discriminated union types

[razor-x]

Summary

• generate per-variant dataclasses and literal-discriminated unions for events and action attempts
• generate discriminated variants for resource error, warning, and other discriminated-object lists
• dispatch known variants during deserialization while preserving unknown API variants as DeepAttrDict
• emit blueprint enums as Literal[...] types when values are available
• update generated routes, webhook parsing, action-attempt polling, and narrowing tests

Compatibility

This changes SeamEvent and ActionAttempt from dataclasses to type aliases over variant dataclasses. It needs an explicit major/minor release decision.

No @seamapi/types bump and no changes to shared merge semantics.

Verification

• npm run typecheck
• npm run lint
• just lint
• just test — 179 passed
• npm run generate twice with an identical diff