Use SHARDS=1 for macOS mbedTLS to stop residual flakiness

The macos-latest runner is consistently slower than ubuntu-latest for
the ASAN+mbedTLS test binary, and SHARDS=2 still flakes there on the
ServerTest fixture's rapid bind/connect cycle against a fixed port.
Serialize fully (SHARDS=1) on macOS only; ubuntu mbedTLS stays at 2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/test.yaml

@@ -234,10 +234,10 @@ jobs:
           LSAN_OPTIONS: suppressions=lsan_suppressions.txt
       - name: build and run tests (Mbed TLS)
         if: matrix.tls_backend == 'mbedtls'
-        # Run mbedTLS shards with reduced parallelism — under ASAN+mbedTLS the
-        # default 4 shards overload CI runners enough that timing-sensitive
-        # ServerTest cases flake on first-request keep-alive reuse.
-        run: cd test && make test_split_mbedtls && SHARDS=2 make test_mbedtls_parallel
+        # macOS runners under ASAN+mbedTLS still flake at SHARDS=2 (rapid
+        # bind/connect on the fixture's fixed port races on the slower
+        # macos-latest runner). Serialize fully here; ubuntu stays at 2.
+        run: cd test && make test_split_mbedtls && SHARDS=1 make test_mbedtls_parallel
       - name: build and run tests (wolfSSL)
         if: matrix.tls_backend == 'wolfssl'
         run: cd test && make test_split_wolfssl && make test_wolfssl_parallel