fix: replace misleading value_prefix_len with actual prefix in token snapshots

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: EffortlessSteven

crates/uselesskey-token/tests/snapshots/snapshots_token__token_api_key_shape.snap

@@ -5,5 +5,5 @@ expression: result
 label: snapshot-api-key
 kind: api_key
 value_len: 40
-starts_with_uk_test: true
+prefix: uk_test_
 authorization_header_prefix: ApiKey

crates/uselesskey-token/tests/snapshots/snapshots_token__token_bearer_shape.snap

@@ -5,5 +5,5 @@ expression: result
 label: snapshot-bearer
 kind: bearer
 value_len: 43
-starts_with_uk_test: false
+prefix: bEHjRLHk
 authorization_header_prefix: Bearer

crates/uselesskey-token/tests/snapshots_token.rs

@@ -14,7 +14,7 @@ struct TokenSnapshot {
     label: &'static str,
     kind: &'static str,
     value_len: usize,
-    starts_with_uk_test: bool,
+    prefix: String,
     authorization_header_prefix: String,
 }
 
@@ -27,7 +27,7 @@ fn snapshot_api_key_shape() {
         label: "snapshot-api-key",
         kind: "api_key",
         value_len: tok.value().len(),
-        starts_with_uk_test: tok.value().starts_with("uk_test_"),
+        prefix: tok.value().chars().take(8).collect::<String>(),
         authorization_header_prefix: tok
             .authorization_header()
             .split(' ')
@@ -48,7 +48,7 @@ fn snapshot_bearer_shape() {
         label: "snapshot-bearer",
         kind: "bearer",
         value_len: tok.value().len(),
-        starts_with_uk_test: tok.value().starts_with("uk_test_"),
+        prefix: tok.value().chars().take(8).collect::<String>(),
         authorization_header_prefix: tok
             .authorization_header()
             .split(' ')