docs(storage): add samples for PrecomputedChecksumsOption

[v-pratap]

This PR updates the examples to demonstrate how to use PrecomputedChecksumsOption.

• Async Sample: Replaced the old UseCrc32cValueOption in storage_async_samples.cc with PrecomputedChecksumsOption.
• Sync Sample: Added InsertObjectWithChecksum and WriteObjectWithChecksum to storage_object_samples.cc to demonstrate providing precomputed hashes to synchronous client calls.

These changes are a follow-up to the previously merged PR that introduced PrecomputedChecksumsOption.

[gemini-code-assist]

Code Review

This pull request updates the Google Cloud Storage C++ examples to demonstrate the use of precomputed checksums when inserting and writing objects. It replaces the deprecated UseCrc32cValueOption with PrecomputedChecksumsOption in async samples and introduces new synchronous examples (InsertObjectWithChecksum and WriteObjectWithChecksum). The review feedback highlights a misleading use of std::move on a const reference string parameter in InsertObjectWithChecksum, which could also lead to an order-of-evaluation bug if the parameter is ever refactored to be passed by value.

[gemini-code-assist]

Using std::move(contents) on a std::string const& is misleading because it does not actually move the string (it results in a copy). More importantly, if contents is ever refactored to be passed by value (e.g., std::string contents) to allow a true move, this code introduces a severe order-of-evaluation bug. Because the order of evaluation of function arguments is indeterminately sequenced, std::move(contents) could be evaluated before gcs::ComputeCrc32cChecksum(contents), causing the checksum to be computed on an empty, moved-from string.

To prevent this, compute the checksum first and store it in a local variable, then pass contents without the misleading std::move.

    auto checksum = gcs::ComputeCrc32cChecksum(contents);
    StatusOr<gcs::ObjectMetadata> object_metadata = client.InsertObject(
        bucket_name, object_name, contents,
        google::cloud::Options{}.set<gcs::PrecomputedChecksumsOption>(
            gcs::PrecomputedChecksums{std::move(checksum), ""}));

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.31%. Comparing base (6f13a6b) to head (c447204).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16308      +/-   ##
==========================================
+ Coverage   92.29%   92.31%   +0.01%     
==========================================
  Files        2226     2226              
  Lines      208573   208851     +278     
==========================================
+ Hits       192504   192792     +288     
+ Misses      16069    16059      -10     

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[kalragauri]

client.InsertObject(...) returns a StatusOr<ObjectMetadata>, and it does not throw exceptions by default when an API error occurs. Because the returned StatusOr is ignored, the bad checksum error will be silently dropped, and the catch block will never execute.

[kalragauri]

Calling std::move on a const reference doesn't move anything; it forces a copy. This appears to be inherited from the original InsertObject snippet, but it is an anti-pattern, so let's avoid it. Here and below.