fix: handle IPv6 host URLs in parseGitLabHostURL

[l-qing]

Problem

On IPv6 GitLab environments, make prepare-gitlab-data fails deterministically and the whole e2e suite space-runs (zero tests, empty Allure report). The failure surfaces in the downstream YAML merge:

Merging gitlab config into .../config.yaml...
Error: Failed to load ./config/users-generated.yaml: yaml: line 3: did not find expected key
make: *** [base.mk: prepare-gitlab-data] Error 1

Root cause

parseGitLabHostURL parsed host:port with a naive split:

host = strings.TrimPrefix(endpoint, "http://")     // "[2335::aa1:1415]:32336"
if strings.Contains(host, ":") {
    parts := strings.Split(host, ":")
    if len(parts) == 2 { host = parts[0]; port = parts[1] }   // only the "exactly one colon" case
}

An IPv6 host has many colons, so len(parts) != 2, the port split is skipped, and host keeps its brackets and port: [2335::aa1:1415]:32336. That value is rendered into users-generated.yaml as:

toolchains:
  gitlab:
    endpoint: http://[2335::aa1:1415]:32336
    host: [2335::aa1:1415]:32336     # value starts with '[' -> YAML flow sequence -> parse error

Because the host: value starts with [, YAML treats it as a flow sequence and the load fails with did not find expected key. IPv4 (10.161.11.29:32739) splits into exactly two parts, so it works — the bug is IPv6-only. The sibling parseGitLabSSHEndpoint in the same file already parses correctly via net/url; this function simply never got the same treatment.

Fix

Rewrite parseGitLabHostURL to parse with net/url:

• url.Hostname() returns the bare host (IPv6 brackets stripped), so the rendered host: scalar is valid YAML (2335::aa1:1415).
• net.JoinHostPort / explicit bracketing re-adds brackets when rebuilding the endpoint, so it stays a valid URL (http://[2335::aa1:1415]:32336).
• Default-port-omitted endpoint behavior is preserved (80/http, 443/https omitted; non-default included).
• Subpath GitLab (https://gitlab.example.com/gitlab) keeps its path.
• No-scheme input (including bare IPv6, with or without a subpath) and malformed URLs fall back without corrupting the endpoint.

Tests

New internal/cli/cmd_test.go:

• TestParseGitLabHostURL — table-driven, 15 cases: IPv6 with/without port (http+https), IPv4 with/without port, hostname, subpath (hostname and IPv6), no-scheme bare IPv6 (with and without subpath), trailing slash, malformed-URL fallback, whitespace trimming.
• TestGeneratedHostValueIsValidYAML — regression guard tying the fix to the original failure: renders the toolchains/gitlab/{endpoint,host} snippet for the IPv6 case and asserts it round-trips through YAML.

go build ./..., go test ./..., and gofmt -l all clean.