fix(healthcheck): support bracketed IPv6 targets and strip brackets before TCP connect

[vaibhavsrv]

Why this matters

In ods/scripts/healthcheck.py, _parse_target() only recognized colon-separated host:port shorthand if the target did not begin with [ (":" in raw and not raw.startswith("[")). As a consequence, valid IPv6 endpoint targets passed in standard bracketed notation (e.g. [::1]:8080) were rejected with ValueError: target must be http(s) URL, tcp://host:port, or host:port. Furthermore, when bracketed IPv6 addresses were parsed from explicit tcp://[::1]:8080 URLs, _parse_host_port() retained the enclosing brackets [::1], which causes standard socket connections (socket.create_connection(("[::1]", port))) to fail with socket.gaierror: [Errno 8] nodename nor servname provided, or not known.

This patch updates _parse_target() to accept bracketed [ipv6]:port shorthand notation alongside standard hostname/IPv4 targets, and ensures _parse_host_port() strips enclosing brackets from IPv6 host addresses prior to opening the TCP socket connection.

Validation

• Repro baseline: Invoked python3 ods/scripts/healthcheck.py [::1]:<port> --timeout 2; confirmed baseline raised ValueError: target must be http(s) URL, tcp://host:port, or host:port. When passed tcp://[::1]:<port>, baseline failed with socket gaierror due to unstripped brackets.
• Post-fix: Verified ods/tests/test_healthcheck_ipv6.py passes with exit code 0; TCP healthcheck successfully connects to listening IPv6 endpoints for both [::1]:port and tcp://[::1]:port targets.
• Scripts healthcheck suites: 1 passed. New-test syntax and diff checks pass; new regressions wired into Linux CI.

Overlap check

• fix(env): probe .env keys literally in _env_set and mode-switch env_set #5819 (0xacee): Addressed macOS environment generation; touches ods/installers/macos/ with zero overlap in scripts/healthcheck.py.
• Probe the live model on the real llama-server host port #5860 (0xacee): Addressed LLM port fallback in build-installation-context.py.
• fix(context): resolve llama-server host port from OLLAMA_PORT and default to 11434 #5863 (vaibhavsrv): Addressed host port resolution in installation context builder.

Risk / AI disclosure

AI-assisted investigation, implementation and CLI regressions. This strengthens target parsing and IPv6 TCP address resolution in the universal healthcheck utility, not runtime daemon execution. Independent human review and platform/runtime qualification remain gates. No running configuration, deployment or upstream merge changed.

Follow-up integration evidence

Composed with #5871 at 81b881c without conflicts. Production and test diffs passed together; network health probe contracts remain intact.
Backlog composition was local-only (production/test diffs, excluding workflow/Makefile wiring); it is not an upstream merge or independent human approval. Declared live-review gates remain open.