http_server: inherit worker ALPN configuration

[edsiper]

Problem

The parent HTTP server configures ALPN on its shared TLS provider before
starting listener workers. Each worker initializes a child server whose local
tls_alpn_configured flag is reset, causing every worker to configure the same
shared OpenSSL SSL_CTX again while sibling workers can already perform TLS
handshakes.

Helgrind identified the concurrent SSL_CTX_set_alpn_select_cb() mutation and
TLS handshake access. Under instrumentation, clients intermittently reported
unexpected message during concurrent HTTP/2 TLS ingestion.

Change

Propagate the parent's ALPN-configured state to each worker server. Workers
continue using the already configured shared TLS provider without mutating it.

Impact

Multi-worker HTTP servers can negotiate HTTP/2 over TLS without racing on
shared ALPN configuration.

This PR is stacked on #12278 because that PR fixes the separate downstream TLS
descriptor lifecycle defect found by the same reproducer.

Validation

• cmake -S . -B build -DFLB_TESTS_RUNTIME=On -DFLB_TESTS_INTERNAL=On
• cmake --build build -j8
• ctest --test-dir build -R 'flb-it-(downstream_worker|http_server|opentelemetry)' --output-on-failure
• Focused OpenTelemetry HTTP/2 TLS worker test: passed normally
• Focused OpenTelemetry HTTP/2 TLS worker test: 3/3 passes with VALGRIND=1 VALGRIND_STRICT=1
• CI-style commit-prefix validation against the stack base

All checks passed.

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