feat: Restore discriminated variants in the Ruby SDK

[razor-x]

Summary

• generate event subclasses by event_type and action-attempt subclasses by action_type
• generate per-code error and warning subclasses while retaining generic fallbacks for unknown discriminants
• dispatch variants from BaseResource.load_from_response without changing shared mergeProperties semantics
• document enum values and correct action-attempt error/result nullability in YARD
• preserve is_a?(SeamEvent) and is_a?(ActionAttempt) compatibility through inheritance
• rename event payload method access to event_method so it no longer shadows Object#method
• update webhook documentation and variant behavior specs

Compatibility

This removes off-variant accessors that previously existed on flattened classes. Although variant instances remain subclasses of the existing base classes, this behavior change should receive a major-version decision before release.

Unknown event types, action types, error codes, and warning codes fall back to their generic base classes for forward compatibility.

Verification

• npm run generate
• npm run typecheck
• npm run lint
• rake lint
• rake test — 224 examples, 0 failures

@seamapi/types remains pinned at 1.1001.0. RBS signatures are intentionally left out of this change.