fix(s3): honor If-Match and If-Unmodified-Since on GetObject - #1446

What broke

S3 GetObject ignores If-Match and If-Unmodified-Since. A read with a stale ETag or an old date gets a 200 (or a 206 for a range) instead of a 412:

GET /s3/bucket/file.txt  If-Match: "stale-etag"                               -> 200
GET /s3/bucket/file.txt  Range: bytes=0-4  If-Match: "stale-etag"             -> 206
GET /s3/bucket/file.txt  If-Unmodified-Since: Sat, 01 Jan 2000 00:00:00 GMT   -> 200

This matters in practice. Recent boto3 and AWS CLI versions (s3transfer) send If-Match on every ranged GET of a multipart download so they can tell when the object changes mid-download. Because we drop the header, an overwrite during a download silently gives you a file that is half the old object and half the new one, with no error.

Why

The route schema only accepted range, if-none-match and if-modified-since, so the other two headers never reached the handler. BrowserCacheHeaders had no field for them, and neither backend checked them.

What changed

• get-object.ts: accept if-match and if-unmodified-since on both GetObject routes. The date goes through the same "ignore invalid dates" parser as If-Modified-Since, which I renamed to parseConditionalDate since it now handles both.
• s3-handler.ts / adapter.ts: pass ifMatch and ifUnmodifiedSince through to the backend.
• S3 backend: forward them as IfMatch / IfUnmodifiedSince. The upstream 412 already maps to a proper S3 error response.
• File backend: check them before the 304 checks, the same way the copy source preconditions already do (If-Unmodified-Since is ignored when If-Match is present), and throw the same 412 PreconditionFailed.

How I tested it

• New integration tests in s3-protocol.test.ts (If-Match mismatch on a ranged GET, If-Unmodified-Since in the past) and file backend unit tests. They fail on master and pass with the fix.
• I also ran real requests against a local server, which now return 412 with the usual S3 error XML:

  --- GET If-Match: "stale-etag"
  <?xml version="1.0" encoding="UTF-8" standalone="yes"?><Error ...><Code>S3Error</Code><Message>At least one of the preconditions you specified did not hold.</Message></Error>
  status 412
  

• s3-protocol.test.ts: 127/128 pass. The one failure (CopyObjectCommand > will not preserve omitted metadata when replacing it) also fails on master for me, because I ran against rustfs locally instead of minio and it defaults the content type differently.
• Unit suite, npm run lint and tsc --noEmit are all clean.