collab: Add has_extended_trial to LlmTokenClaims (#29622)

This PR adds the `has_extended_trial` field to the LLM token claims.

Release Notes:

- N/A

crates/collab/src/llm.rs

@@ -5,6 +5,8 @@ use crate::Cents;
 
 pub use token::*;
 
+pub const AGENT_EXTENDED_TRIAL_FEATURE_FLAG: &str = "agent-extended-trial";
+
 /// The maximum monthly spending an individual user can reach on the free tier
 /// before they have to pay.
 pub const FREE_TIER_MONTHLY_SPENDING_LIMIT: Cents = Cents::from_dollars(10);