Drop mbedTLS continue-on-error now that the matrix is stable

With the close_notify mid-response fix and SHARDS=2 mitigation, the
mbedTLS legs run reliably on both ubuntu and macos. Drop the
continue-on-error escape hatch so future regressions actually break the
build.

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

.github/workflows/test.yaml

@@ -80,10 +80,6 @@ jobs:
       fail-fast: false
       matrix:
         tls_backend: [openssl, mbedtls, wolfssl]
-    # TODO: mbedTLS jobs hit pre-existing flaky failures (timing-sensitive
-    # ServerTest cases under ASAN+mbedTLS). Allow them to fail without
-    # blocking until the underlying flakiness is investigated.
-    continue-on-error: ${{ matrix.tls_backend == 'mbedtls' }}
     name: ubuntu (${{ matrix.tls_backend }})
     steps:
       - name: checkout
@@ -221,8 +217,6 @@ jobs:
       fail-fast: false
       matrix:
         tls_backend: [openssl, mbedtls, wolfssl]
-    # See ubuntu job above.
-    continue-on-error: ${{ matrix.tls_backend == 'mbedtls' }}
     name: macos (${{ matrix.tls_backend }})
     steps:
       - name: checkout