health_check: gracefully handle upstream connection creation failure

[bpalermo]

Commit Message

health_check: gracefully handle upstream connection creation failure

Additional Description

Follow-up to #45975, which made HostImplBase::createConnection return a null connection
(instead of crashing) when the upstream connection cannot be created — for example when binding
to a configured Linux network namespace (SocketAddress.network_namespace_filepath) that became
unavailable at runtime. The connection pools were taught to surface that as a pool failure, but
the active health checkers create connections directly via createHealthCheckConnection and
still dereference the result unconditionally, so they crash in the same scenario.

This change makes the TCP, HTTP, gRPC, and Thrift active health checkers treat a null connection
as a NETWORK health-check failure instead of dereferencing it. The TCP and gRPC onTimeout
handlers also tolerate a null client, because the interval base re-arms the timeout timer after a
failed interval. Thrift's Client::start now returns whether the connection was created so the
session can fail gracefully.

The redis proxy upstream client has the same direct-call pattern; making it graceful requires
threading a connection-creation failure through the redis connection pool and is left as a
follow-up.

Risk Level

Low — converts a crash into the existing graceful network-failure path.

Testing

Unit tests: a TCP health checker test covering a null connection and the re-armed timeout firing
against a session with no client (test/common/upstream/health_checker_impl_test.cc); Thrift
mock updated for the new Client::start return value.

Docs Changes

N/A

Release Notes

Included (changelogs/current/bug_fixes/sockets__health_check_network_namespace.rst).

Platform Specific Features

N/A

[bpalermo]

@yanavlasov could you take a look? This is the follow-up to #45975 (which you reviewed): that PR made HostImplBase::createConnection return a null connection instead of crashing and taught the connection pools to handle it, but the active health checkers (TCP, HTTP, gRPC, Thrift) create connections directly and still dereference the result unconditionally. This applies the same graceful handling there, turning the null connection into a NETWORK health-check failure.