collab: Make account age-related fields required in LlmTokenClaims (#26959)

This PR makes the account age-related fields required in
`LlmTokenClaims`.

We've also removed the account age check from the LLM token issuance
endpoint, instead having it solely be enforced in the `POST /completion`
endpoint.

This change will be safe to deploy at ~8:01PM EDT.

Release Notes:

- N/A

crates/collab/src/llm/token.rs

@@ -20,17 +20,10 @@ pub struct LlmTokenClaims {
     pub system_id: Option<String>,
     pub metrics_id: Uuid,
     pub github_user_login: String,
-    // This field is temporarily optional so it can be added
-    // in a backwards-compatible way. We can make it required
-    // once all of the LLM tokens have cycled (~1 hour after
-    // this change has been deployed).
-    #[serde(default)]
-    pub account_created_at: Option<NaiveDateTime>,
+    pub account_created_at: NaiveDateTime,
     pub is_staff: bool,
     pub has_llm_closed_beta_feature_flag: bool,
-    #[serde(default)]
     pub bypass_account_age_check: bool,
-    #[serde(default)]
     pub has_predict_edits_feature_flag: bool,
     pub has_llm_subscription: bool,
     pub max_monthly_spend_in_cents: u32,
@@ -65,7 +58,7 @@ impl LlmTokenClaims {
             system_id,
             metrics_id: user.metrics_id,
             github_user_login: user.github_login.clone(),
-            account_created_at: Some(user.account_created_at()),
+            account_created_at: user.account_created_at(),
             is_staff,
             has_llm_closed_beta_feature_flag: feature_flags
                 .iter()