tokenGateController.ts: Add support for time-bounded token gate windows

[Kingsman-99]

Description

src/tokenGateController.ts evaluates token gate conditions based on balance thresholds only. Some use cases require gates that are only active during a specific time window (e.g., early-access periods). Adding validFrom and validUntil options to gate configs enables time-bounded access control.

Acceptance Criteria

• TokenGateConfig accepts optional validFrom?: Date and validUntil?: Date fields
• A gate evaluation outside the time window returns false regardless of balance
• A gate without time constraints evaluates balance only (no behavior change)
• Unit tests pass

Context

• Target file: src/tokenGateController.ts
• Use src/timeMachine.ts in tests to simulate time boundaries