post-buffered-inline-comments exits 0 after failing to post every comment (Posted 0/N goes green)

## Summary

When `createReviewComment` fails for a buffered comment, `post-buffered-inline-comments.ts` swallows the error and continues. If **every** comment fails, the step logs `Posted 0/N`, exits 0, and the job goes green. A review that dropped 100% of its findings is indistinguishable from a review that found nothing.

## Current code (`d721746`)

[`postComment` :135-143](https://github.com/anthropics/claude-code-action/blob/d721746d683d812e669ce117cebe55a85fbd9c3e/src/entrypoints/post-buffered-inline-comments.ts#L135-L143) — failure is logged at `console.log` (not even `::error::`) and reported as a boolean:

```ts
} catch (e) {
console.log(` failed ${c.path}:${c.line}: ...`);
return false;
}
```

[`main` :219-227](https://github.com/anthropics/claude-code-action/blob/d721746d683d812e669ce117cebe55a85fbd9c3e/src/entrypoints/post-buffered-inline-comments.ts#L219-L227) — the count is printed and discarded:

```ts
let posted = 0;
for (const c of toPost) {
if (await postComment(...)) { posted++; }
}
console.log(`Posted ${posted}/${toPost.length}`);
```

`posted` is never compared to `toPost.length`. `main()` returns normally, so the `always()` step succeeds.

## Observed

2026-08-16: **40 consecutive runs** logged `Posted 0/N` and reported success. Every post was rejected with `422 pull_request_review_thread.path/line could not be resolved`. Nothing in the check status, the PR, or the job summary indicated the reviewer had delivered nothing — we only found it by reading raw step logs.

Our 422s came from #1542 (stale cross-PR buffer entries whose paths aren't in the current diff), but the silent-success is independent of that cause. Any 422 — a line outside the diff, a stale `commit_id` after a force-push, an outdated `headSha` — produces the same green-but-empty result. It's also the failure mode most likely to persist *after* #1542 is fixed, since a legitimately mis-anchored comment still vanishes silently.

## Contrast with the sibling failure path

#1667 covers the opposite over-reaction in the same file: one malformed buffer line throws out of `main()` and exits 1, discarding everything. So the module currently hard-fails on a parse error it could recover from, and silently succeeds on a delivery failure that loses user-visible output. Both paths deserve the middle ground.

## Suggested fix

1. Emit `::error::` (not `console.log`) per failed post, including the API error body — a 422 naming the unresolvable path is the single most useful diagnostic and it's currently buried.
2. When `posted < toPost.length`, either exit non-zero or emit a `::error::` summary, so a review that dropped findings is visibly distinct from a clean one.
3. Print the body of each dropped comment so the finding survives in the log even when it can't be anchored.

Point 2 is arguably a behaviour change for anyone relying on the step never failing, so a warning-only variant (`::error::` without a non-zero exit) would still fix the invisibility. Happy to open a PR either way.

## Environment

- `anthropics/claude-code-action@v1`, source read at `d721746d683d812e669ce117cebe55a85fbd9c3e`
- Self-hosted runners; `classify_inline_comments` default (on), `ANTHROPIC_API_KEY` unset (OAuth token auth)