fix: update 6 test files broken by dead code removal

- test_percentage_clamp.py: remove TestContextCompressorUsagePercent class
  and test_context_compressor_clamped (tested removed get_status() method)
- test_credential_pool.py: remove test_mark_used_increments_request_count
  (tested removed mark_used()), replace active_lease_count() calls with
  direct _active_leases dict access, remove mark_used from thread test
- test_session.py: replace SessionSource.local_cli() factory calls with
  direct SessionSource construction (local_cli classmethod removed)
- test_error_classifier.py: remove test_is_transient_property (tested
  removed is_transient property on ClassifiedError)
- test_delivery.py: remove TestDeliveryRouter class (tested removed
  resolve_targets method), clean up unused imports
- test_skills_hub.py: remove test_is_hub_installed (tested removed
  is_hub_installed method on HubLockFile)

tests/agent/test_error_classifier.py

@@ -75,28 +75,6 @@ class TestClassifiedError:
         e3 = ClassifiedError(reason=FailoverReason.billing)
         assert e3.is_auth is False
 
-    def test_is_transient_property(self):
-        transient_reasons = [
-            FailoverReason.rate_limit,
-            FailoverReason.overloaded,
-            FailoverReason.server_error,
-            FailoverReason.timeout,
-            FailoverReason.unknown,
-        ]
-        for reason in transient_reasons:
-            e = ClassifiedError(reason=reason)
-            assert e.is_transient is True, f"{reason} should be transient"
-
-        non_transient = [
-            FailoverReason.auth,
-            FailoverReason.billing,
-            FailoverReason.model_not_found,
-            FailoverReason.format_error,
-        ]
-        for reason in non_transient:
-            e = ClassifiedError(reason=reason)
-            assert e.is_transient is False, f"{reason} should NOT be transient"
-
     def test_defaults(self):
         e = ClassifiedError(reason=FailoverReason.unknown)
         assert e.retryable is True