Compose target stage completion across multiple lines raises an error

[rcjsuen]

Using code completion on a different line from services.[N].build.target can cause a panic.

services:
  test:
    build:
      target: abc
        

{
    "textDocument": {
        "uri": "file:///tmp/compose.yaml"
    },
    "position": {
        "line": 4,
        "character": 8
    },
    "context": {
        "triggerKind": 1
    }
}

goroutine 52 [running]:
runtime/debug.Stack()
	/opt/hostedtoolcache/go/1.23.8/x64/src/runtime/debug/stack.go:26 +0x64
runtime/debug.PrintStack()
	/opt/hostedtoolcache/go/1.23.8/x64/src/runtime/debug/stack.go:18 +0x1c
github.com/docker/docker-language-server/internal/pkg/server.(*Server).handleRecovered(0x140005b0000, {0x1400051e660, 0x17}, {0x10180a520, 0x1400051e6a8})
	/home/runner/work/docker-language-server/docker-language-server/internal/pkg/server/server.go:199 +0x44
github.com/docker/docker-language-server/internal/pkg/server.NewServer.func1({0x1400051e660?, 0x140000573c7?}, {0x10180a520?, 0x1400051e6a8?})
	/home/runner/work/docker-language-server/docker-language-server/internal/pkg/server/server.go:127 +0x40
github.com/docker/docker-language-server/internal/tliron/glsp/protocol.(*Handler).Handle.func1()
	/home/runner/work/docker-language-server/docker-language-server/internal/tliron/glsp/protocol/handler.go:100 +0x6c
panic({0x10180a520?, 0x1400051e6a8?})
	/opt/hostedtoolcache/go/1.23.8/x64/src/runtime/panic.go:791 +0x124
github.com/docker/docker-language-server/internal/compose.buildTargetCompletionItems(0x14000672f90, 0x140000d58f0, {0x1400067a5a0, 0x4, 0x9?}, 0x1400027f320, 0x0)
	/home/runner/work/docker-language-server/docker-language-server/internal/compose/completion.go:348 +0x398
github.com/docker/docker-language-server/internal/compose.Completion({0x102230d10?, 0x10181fd00?}, 0x14000672f90, 0x140000d58f0, {0x1018d2360, 0x14000319ce0})
	/home/runner/work/docker-language-server/docker-language-server/internal/compose/completion.go:158 +0x2e0
github.com/docker/docker-language-server/internal/pkg/server.(*Server).TextDocumentCompletion(0x140005b0000, 0x14000674730, 0x14000672f90)
	/home/runner/work/docker-language-server/docker-language-server/internal/pkg/server/completion.go:22 +0x254
github.com/docker/docker-language-server/internal/tliron/glsp/protocol.(*Handler).Handle(0x140006a6008, 0x14000674730)
	/home/runner/work/docker-language-server/docker-language-server/internal/tliron/glsp/protocol/handler.go:388 +0x146c
github.com/docker/docker-language-server/internal/tliron/glsp/server.(*Server).handle(0x140001ba1e0, {0x1018c9988, 0x140003dc070}, 0x140005b6000, 0x14000670c00)
	/home/runner/work/docker-language-server/docker-language-server/internal/tliron/glsp/server/handler.go:48 +0x220
github.com/sourcegraph/jsonrpc2.(*HandlerWithErrorConfigurer).Handle(0x1400051a3c0, {0x1018c9988, 0x140003dc070}, 0x140005b6000, 0x14000670c00)
	/home/runner/go/pkg/mod/github.com/sourcegraph/jsonrpc2@v0.2.0/handler_with_error.go:21 +0x4c
github.com/sourcegraph/jsonrpc2.(*Conn).readMessages(0x140005b6000, {0x1018c9988, 0x140003dc070})
	/home/runner/go/pkg/mod/github.com/sourcegraph/jsonrpc2@v0.2.0/conn.go:205 +0x2a4
created by github.com/sourcegraph/jsonrpc2.NewConn in goroutine 1
	/home/runner/go/pkg/mod/github.com/sourcegraph/jsonrpc2@v0.2.0/conn.go:62 +0x1d0