[CK][CK_TILE] Fix dispatcher cpp tests - registry key mismatch and string assertions (#6528)

## Motivation

CPP tests in dispatcher were failing due to a mismatch in registry key
and string representation.

## Technical Details
Bug 1 - Registry key mismatch: The registry stored kernels using
get_name() but lookups used encode_identifier(), causing all registry
lookups to fail. Fixed by changing registry.cpp:58 to use
encode_identifier() for storage.
Bug 2 - String representation changes: Tests checked for
"persist"/"nopers" substrings, but the code emits "True"/"False". Fixed
by replacing brittle substring checks with comparison-based assertions
in test_kernel_key.cpp and test_kernel_key_extended.cpp.

## Test Plan

Tested with CPP tests in dispatcher 

## Test Result

Validation: All three core cpp tests now pass:
  - test_kernel_key - 6/6 tests passing
  - test_kernel_key_extended - 25/25 tests passing
  - test_registry - 8/8 tests passing
  
 
## Submission Checklist

- [ x] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

dispatcher/tests/test_tile_backend.cpp

@@ -97,8 +97,10 @@ TEST(TileBackendTest, TileKernelIdentifierEncoding)
     EXPECT_NE(id.find("2x2x1"), std::string::npos);
     EXPECT_NE(id.find("32x32x16"), std::string::npos);
 
-    // Should contain persistent flag
-    EXPECT_NE(id.find("nopers"), std::string::npos); // persistent = false
+    // Verify persistent flag affects identifier
+    KernelKey persistent_key            = key;
+    persistent_key.algorithm.persistent = true;
+    EXPECT_NE(id, persistent_key.encode_identifier());
 }
 
 TEST(TileBackendTest, MultipleKernelRegistration)