add host_override to parent.config and other sni name fixes

[traeak]

Fixes to sni name handling when using another CDN as parent.

This adds "host_override" to parent.config and has some fixes for the strategies handling of same option.

[Copilot]

Pull request overview

This PR updates parent/SNI handling so ATS can override the outbound Host/SNI name when using a parent (e.g., another CDN), and adds a gold test + TLS fixtures to validate parent failover behavior.

Changes:

• Add host_override support to parent.config parsing and expose it on ParentResult.
• Update Host header override behavior in parent selection / next-hop strategy paths, and harden SNI server_name handling.
• Add a TLS parent-failover gold test plus associated cert/key fixtures and helper scripts.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/gold_tests/tls/valgrind.sh	Helper script to run Traffic Server under valgrind for TLS gold tests.
tests/gold_tests/tls/tls_sni_parent_failover.test.py	New gold test for SNI/Host behavior during parent failover with host override.
tests/gold_tests/tls/ssl/server-foo.pem	TLS cert fixture for foo.com origin.
tests/gold_tests/tls/ssl/server-foo.key	TLS private key fixture for foo.com origin.
tests/gold_tests/tls/ssl/server-bar.pem	TLS cert fixture for bar.com origin.
tests/gold_tests/tls/ssl/server-bar.key	TLS private key fixture for bar.com origin.
tests/gold_tests/tls/ssl/gen_foobar_certs.sh	Script for regenerating the foo/bar TLS fixtures used by the new test.
src/proxy/http/remap/NextHopSelectionStrategy.cc	Adjust Host header override to target the correct request header depending on state.
src/proxy/http/HttpTransact.cc	Apply Host override when parent.config selects a parent with host_override=true.
src/proxy/http/HttpSM.cc	Null/empty handling fix for outbound SNI when policy is server_name.
src/proxy/ParentSelection.cc	Parse host_override from parent.config.
include/proxy/http/remap/NextHopSelectionStrategy.h	Remove redundant include (via HttpTransact) after new host_override handling.
include/proxy/ParentSelection.h	Add host_override flag to ParentRecord and accessor on ParentResult.

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