fix: joinUrl no longer collapses the URL scheme

[vsaraikin]

What

joinUrl collapses the // in a URL scheme, so it produces an invalid URL:

vars:
  SERVER: "http://localhost"
tasks:
  default:
    cmds:
      - echo "{{joinUrl .SERVER "path1" "path2"}}"
      # got:  http:/localhost/path1/path2
      # want: http://localhost/path1/path2

The documented behavior (docs/reference/templating.md, "URLs" section) shows http://localhost/path1/path2, so this is a clear bug — and it's the whole reason joinUrl exists separately from joinPath.

Why

joinUrl was implemented as path.Join(elem...). path.Join runs path.Clean, which collapses consecutive slashes and destroys the // after the scheme.

Fix

Use net/url.JoinPath, which is purpose-built for joining URL path segments and preserves the scheme. The signature becomes (string, error), consistent with the sibling relPath/absPath funcs (filepath.Rel/filepath.Abs), and the template engine already supports funcs that return an error.

Tests

Added TestJoinUrl in internal/templater/funcs_test.go covering scheme preservation, a base with an existing path, and trailing-slash handling. joinUrl shipped without a test in #2408, which is how this slipped through.

[trulede]

Seems OK.

[vmaerten]

Thanks!
Can you fix the lint ? https://github.com/go-task/task/actions/runs/29262496919/job/86931062202?pr=2915

[vsaraikin]

Done — added t.Parallel() to TestJoinUrl and its subtests to satisfy the paralleltest linter. Thanks!