fix(core): merge per-call restOptions with the client-level ones

[smichaelsen]

Fixes #9212.

Problem

RequestWrapper::getRequestOptions() replaces the client-level restOptions with the per-call ones whenever a call supplies any:

$restOptions = $options['restOptions'] ?? $this->restOptions;

Transport settings configured once on the client — proxy, verify, cert — are therefore silently dropped for any call that passes its own restOptions.

Storage made this visible in v1.51.0. The X-Goog-Hash checksum header added in #8825 travels as a per-call restOptions entry, so every upload loses the client's proxy configuration. Downloads reach the same code path through the on_headers callback used to detect transcoded objects. In proxy-only environments uploads and downloads stop working, while metadata and auth calls — which pass no per-call restOptions — keep succeeding, which makes the failure look unrelated to the upgrade.

This is not Storage-specific: any component that sets per-call restOptions drops client transport configuration the same way.

Fix

Merge instead of replace. Per-call options win on conflict, and nested headers arrays are merged on their own so a per-call header does not drop the client's default headers.

The merge is deliberately shallow otherwise. array_merge_recursive is not usable here: Guzzle options mix scalars and arrays, and colliding scalars such as proxy would be turned into arrays that Guzzle rejects.

Tests

Three cases added to Core/tests/Unit/RequestWrapperTest.php:

• client-level proxy/verify survive a call that supplies its own restOptions (the Storage upload scenario)
• per-call options take precedence over client-level ones on conflict
• headers merge rather than replace

The first two fail on main with Undefined array key "proxy". The full Core unit suite passes (534 tests), as does phpcs --standard=phpcs-ruleset.xml on both changed files.

Credit

The root cause analysis and the merge strategy are @salilg-eng's, from the discussion in #9212. I'm opening the PR since the issue has been sitting for a couple of months and it is currently blocking us from upgrading past google/cloud-storage 1.50.

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[bshaffer]

I have one minor suggestion, but in general this is LGTM. We should get a review from @thiyaguk09 as well

[salilg-eng]

Looks good to me! This is exactly what I had in mind for #9212. The array union approach is clean, and handling the headers separately was a good call.

[bshaffer]

@smichaelsen thank you for your contribution! Please sign the github CLA and we can merge your changes.