Support conditional writes (If-Match / If-None-Match) on PutObject and CopyObject

[Robothy]

Closes #289

What

Adds support for the S3 conditional write headers on PutObject, CopyObject and CompleteMultipartUpload:

• If-Match -- write only if the current ETag matches (* requires the object to exist)
• If-None-Match -- write only if the current ETag does not match (* requires the object to be absent)

Returns 412 PreconditionFailed when a condition does not hold. Reuses the existing S3ErrorCode.PreconditionFailed, no new error code.

How

The check happens in a single place -- PutObjectService#putObject, before content is written to storage, so a failed precondition never leaves a dangling object. CopyObject and CompleteMultipartUpload both route through it and pass the headers along.

ETag comparison tolerates both quoted and unquoted values, because clients echo back the ETag returned by the server verbatim (the server returns "<hex>").

Tests

New ConditionalRequestIntegrationTest with 8 cases covering wildcard and explicit ETags, match/mismatch on both headers, quoted ETags from the server, CopyObject, and 404 precedence on a missing bucket.

Full suite: 640 tests, 0 failures (baseline 632 + 8 new).

[codecov]

Codecov Report

❌ Patch coverage is 89.74359% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.75%. Comparing base (9b2a02c) to head (70451f1).

Files with missing lines	Patch %	Lines
.../robothy/s3/core/util/ConditionalRequestUtils.java	81.81%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #294      +/-   ##
============================================
- Coverage     90.76%   90.75%   -0.01%     
- Complexity     1459     1479      +20     
============================================
  Files           248      250       +2     
  Lines          5327     5365      +38     
  Branches        399      407       +8     
============================================
+ Hits           4835     4869      +34     
- Misses          338      340       +2     
- Partials        154      156       +2     

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