Add support for Guzzle 8

[const-cloudinary]

Brief Summary of Changes

Allow Guzzle 8 alongside Guzzle 7: guzzlehttp/guzzle ^7.4.5|^8.0, guzzlehttp/promises ^1.5.3|^2.0|^3.0, guzzlehttp/psr7 ^2.7|^3.0. Existing Guzzle 7 users are unaffected; Guzzle 7 remains in maintenance upstream.

Code changes needed for Guzzle 8 (all also apply on Guzzle 7):

• ApiClient::buildHttpClientConfig: send the auth option only when both api_key and api_secret are set. Guzzle 8 rejects null auth values. This also fixes unsigned upload when no credentials are configured, which currently fails with 401 on Guzzle 7 as well.
• BaseApiClient::callAsync: cast request header values to strings (Guzzle 8 rejects non-string values) and skip unnamed entries. The list form of the headers upload parameter (['X-Robots-Tag: noindex']) is still sent to the API as the parameter, but is no longer added to the HTTP request headers (Guzzle 7 sent it as a header named 0; Guzzle 8 throws).
• BaseApiClient::callAsync: the rejection handler accepts any Throwable, since Guzzle 8 also rejects with non-Exception errors.
• ApiConfig::$connectionTimeout: docblock describes the Guzzle 7 / Guzzle 8 defaults.
• CI: matrix now runs every PHP version against both Guzzle 7 and Guzzle 8.

Behavior changes coming from Guzzle 8 itself, for the release notes:

• Connect timeout defaults to 60 seconds when connection_timeout is not set (libcurl default was 300).
• Read timeouts are reported as NetworkTimeoutException, which is not a ConnectException.
• Multipart parts no longer include a per-part Content-Length header (RFC 7578 §4.8).
• Apps that get Guzzle only through this SDK will move to Guzzle 8 on composer update; to stay on 7: composer require guzzlehttp/guzzle:^7.

What does this PR address?

• GitHub issue (Add reference - Support Guzzle 8 #429)
• Refactoring
• New feature
• Bug fix
• Adds more tests

Are tests included?

• Yes
• No

Reviewer, please note:

• AssetsTest::testVisualSearchImageFile asserted the per-part Content-Length that psr7 3 no longer generates. It now checks the image_file multipart option directly (exact file bytes and filename), which works on both majors.
• Related: Drop Guzzle requirement and replace with any PSR-18 HttpClient #396 (PSR-18 support) is not addressed here.

Checks from #429

• Header values must be strings: header values from headers/extra_headers are cast to strings in BaseApiClient::normalizeHeaders; SDK-set headers (User-Agent, Authorization, X-Unique-Upload-Id, Content-Range) were already strings.
• Request method casing: no change needed. All requests go through requestAsync() with uppercase constants from HttpMethod.
• psr7 3.x upgrade guide: changes visible through the SDK: per-part multipart Content-Length is no longer sent (see above); ", CR and LF in multipart file names are escaped as %22, %0D, %0A; Uri rejects a few more malformed URLs. LimitStream and Utils::streamFor behavior used by chunked upload is unchanged; UploadApiTest::testUploadLarge covers it in CI.
• promises 3.x upgrade guide: *Async() return types are unchanged. One behavior change: a rejection handler attached to an SDK promise after the request has already failed is now invoked (promises 2 skipped it).

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.

Closes #429

🤖 Generated with Claude Code