feat: Slight performance improvements

[kissifrot]

Brief Summary of Changes

Several small performance improvements targeting the most common hot paths: asset construction, URL generation, and API requests. No behaviour change.

• BaseAsset::configuration() and AuthToken::configuration(): added a fast path that clones config sections directly when a Configuration instance is passed, avoiding a full new Configuration(...) rebuild (7-object init + serialize/deserialize cycle) on every asset creation.
• Configuration::jsonSerialize() and BaseAsset::jsonSerialize(): replaced array_merge($json, $section) in loops with the union operator +=, equivalent for unique string-keyed arrays.
• ApiClient::buildHttpClientConfig(): replaced array_merge_recursive with an explicit header merge + array_merge, avoiding nested-array side effects and unnecessary overhead.
• AssetFinalizerTrait::finalizeSource() / finalizeVersion(): replaced preg_match calls with stripos/strncmp (preserving original case-sensitivity per call site); eliminated a redundant double call to publicId() in finalizeVersion().
• AssetDescriptor::setSuffix(): replaced preg_match('/[.\/]/') with str_contains.

What does this PR address?

• GitHub issue (Add reference - #XX)
• Refactoring
• New feature
• Bug fix
• Adds more tests

Are tests included?

• Yes
• No

Reviewer, please note:

• BaseAsset::configuration() fast path uses clone instead of a serialize/deserialize round-trip. Independence from the source Configuration singleton is covered by the new testAssetConfigurationIsIndependentFromGlobalConfig test.
• The finalizeSource() regex /^https?:\//i matches a single slash (e.g. http:/), not ://. The stripos replacement preserves this exact semantics.
• finalizeVersion() used a case-sensitive regex; strncmp preserves that, while finalizeSource() used /i; stripos preserves that.
• OAuth tests are extended to assert that User-Agent is preserved alongside Authorization — this is the regression guard for the buildHttpClientConfig() change.

Edit: I added a benchmark tool, below are the results:
10 000 iterations × 3 runs averaged on each branch (php tools/benchmark.php).

Scenario	Before (master)	After	Gain
new Image($source)	1238 ms	21 ms	x59.9
(string) new Image($source)	1417 ms	155 ms	x9.1
$image->toUrl() (pre-built asset)	152 ms	132 ms	-13%
new Image + setSuffix()	1409 ms	148 ms	x9.5
(string) new Video($source)	1406 ms	153 ms	x9.2
Configuration::jsonSerialize()	296 ms	290 ms	-2%
new Image($source, $configArray) (array slow path)	648 ms	637 ms	-2%

Main gain comes from the configuration() fast path: asset construction no longer triggers a full new Configuration(...) rebuild (7-object init + serialize/deserialize cycle) on every call.
The toUrl() row on a pre-built asset isolates URL generation improvements independently of construction overhead.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I ran the full test suite before pushing the changes and all of the tests pass.