Increase test coverage by 16.82% and fix _determineLength TypeError with neverSerializePayload

[Copilot]

Line coverage was at 72.41%. Added 191 new tests across three files, raising coverage to 89.23% (+16.82pp). In the process, tests uncovered a real TypeError bug in production code.

Bug fix: Request::_determineLength() crashes with neverSerializePayload()

When calling neverSerializePayload() on a request with a string body, the payload is stored internally as a single-element array. _serializePayload with SERIALIZE_PAYLOAD_NEVER returns it unchanged, then strlen(array) throws a TypeError.

// Before fix — throws TypeError: strlen(): Argument #1 must be of type string, array given
Request::post('https://api.example.com/', 'raw body')->neverSerializePayload()->send();

Fixed _determineLength to handle non-string payloads gracefully:

// Before
return \strlen($str);

// After
return \strlen((string) $str);  // also guards against array with early return

New test coverage

• ExtraCoverageTest.php — Request builder/auth/PSR-7 methods, Uri, Headers, Stream, MIME handlers (CsvMimeHandler, XmlMimeHandler), UploadedFile, exception classes
• ExtraCoverageExtendedTest.php — Curl\Curl getters/setters/callbacks (no network), UriResolver static helpers (unparseUrl, relativize, removeDotSegments), Client *_request builders, all uncovered Response methods, Http/Mime helpers
• ClientMultiAndPromiseTest.php — ClientMulti all add_* methods (curl prep only, no HTTP), MultiCurl configuration, MultiCurlPromise then()/wait()/getState(), Request::_curlPrep() across all branching paths (auth, timeout, redirects, protocol versions, proxy, download, keep-alive, etc.)

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This change is 

[coveralls]

coverage: 89.228% (+16.8%) from 72.405% — copilot/increase-test-coverage-15-percent into master