runtime/docker: prePullBaseImages discards the pull error, so a transient registry failure surfaces as "no active sessions"

## Symptom

An integration test that builds an image fails with:

```
build failed: alpine:3.20: failed to resolve source metadata for docker.io/library/alpine:3.20: no active sessions
```

Observed on `test-integration-linux (1.26, 2)` in PR #129 on `TestEngineBuild_NoContainerCreated`. The same test passed on Go 1.25 in the same shard of the same run, and passed on both versions on the two following heads — so the input that varied was the registry, not the code.

The message is misleading. It names BuildKit's session requirement, which is working exactly as designed and is not what went wrong.

## Cause

`BuildImage` pre-pulls base images specifically so BuildKit never has to resolve remote metadata (`runtime/docker/build.go`, the comment above the `prePullBaseImages` call spells this out: BuildKit refuses remote resolution without an active session, so the classic `ImagePull` API seeds the local store and BuildKit picks up the cached manifest).

The pre-pull is best-effort and the error is discarded:

```go
for _, ref := range extractBaseImages(string(df), buildArgs) {
if _, err := r.api.ImageInspect(ctx, ref); err == nil {
continue
}
_, _ = r.PullImage(ctx, ref, events)
}
```

So any transient pull failure — docker.io rate limiting, a DNS blip, a registry 5xx — leaves the local store unseeded, and the build then fails several layers away with BuildKit's session error. The real cause (the pull, and why it failed) is never reported anywhere.

This is what makes the flake expensive: the error points a reader at BuildKit and at session handling, and the first hypothesis on a dependency-bump branch is that the Docker client bump broke the build path. Ruling that out took a diff of `moby/moby/client` 0.4.1 → 0.5.1 to confirm `image_build.go`, `image_pull.go` and session handling were all untouched.

## Suggested fix

Keep the pre-pull best-effort — it is legitimately advisory, since `extractBaseImages` deliberately drops refs it cannot resolve and a pull failure is not always fatal — but stop throwing the error away. Either:

- retain the last pull error per ref and, when `ImageBuild` subsequently fails, wrap it so the build error carries the pre-pull failure that likely caused it; or
- emit the pull failure as a `BuildEvent` warning at the point it happens, so it lands in the build log ahead of the confusing error.

The first gives the better diagnostic (the two errors appear together, causally ordered) and does not add per-run noise for the benign case where the ref was already local or the pull failure did not matter.

## Relationship to #50

#50 deletes `prePullBaseImages` and `extractBaseImages` outright by opening a real BuildKit session with an auth provider, which removes this failure mode along with the workaround. That is the right end state, and its §2 notes a neighbouring case — a ref the parser cannot resolve falls through to the same "no active sessions" error.

This issue is narrower and worth doing independently: it is about the case where the parser resolved the ref correctly and the *pull* failed, and the fix is a few lines rather than a new direct dependency on `github.com/moby/buildkit`. If #50 lands first this becomes moot; until then, CI flakes here misdiagnose themselves.

## Severity

Low, but it costs real time on every occurrence: a red CI job whose message points away from the actual cause.