fix(workspace_booking): address type safety and error coverage gaps

[Kaylahray]

fix(workspace_booking): address type safety and error coverage gaps

• Change hourly_rate and amount_paid from i128 to u128
• Add NoShow and Expired variants to BookingStatus
• Add Virtual and Hybrid variants to WorkspaceType
• Replace is_available bool with WorkspaceAvailability enum (carries UnavailabilityReason)
• Add MAX_ID_LEN and MAX_NAME_LEN constants; document length constraints on id and name fields
• Add cancelled_at and completed_at timestamps to Booking
• Add BookingExpired, InsufficientBalance, BookingAlreadyCancelled, BookingAlreadyCompleted, and WorkspaceHasActiveBookings error variants
• Introduce reserved error code ranges (core 1–99, booking 100–199, workspace 200–299)

Closes #614

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