PHP SDK: production quality & feature parity tracking

[hubcio]

Umbrella tracking issue for the experimental PHP SDK (foreign/php/).

Initial implementation lands in #3235.

Goal: bring the PHP SDK to the same implementation quality and feature parity as the other foreign SDKs (Python, Node, Go, Java, C#, C++).

Sub-issues

CI / release pipeline:

• PHP SDK: wire release pipeline #3304 - wire release pipeline (add sdk-php to publish.yml, add .github/actions/php/post-merge/action.yml, include sdk-php in the SDK loop in post-merge.yml, decide and wire PECL package.xml and/or Packagist).
• PHP SDK: pin supported PHP version in CI and package metadata #3305 - pin supported PHP version in CI and package metadata (install php8.3-cli, php8.3-dev, php8.3-mbstring, php8.3-xml explicitly, tighten composer.json with "require": {"php": ">=8.3"}, document supported PHP version in foreign/php/README.md).
• PHP SDK: add coverage, test reports, and Codecov support #3307 - add coverage instrumentation, JUnit test reports, and Codecov upload (wrap the PHP test-mode cargo build with cargo llvm-cov, emit lcov, produce JUnit XML, upload via actions/upload-artifact, wire codecov).

Packaging:

• PHP SDK: fill out Packagist metadata #3303 - fill out Packagist metadata in composer.json (autoload, keywords, authors, homepage).

Parity with the other SDKs:

• PHP SDK: add BDD scenarios and bdd-php component #3310 - add bdd/php/ scenarios and a bdd-php component in .github/config/components.yml.
• PHP SDK: add examples and examples-php component #3311 - add examples/php/ and an examples-php component.

API:

• Map PHP SDK errors to typed exception classes #3294 - map PHP SDK errors to typed exception classes.
• Add an iterator analogue to Python's IggyConsumer.iter_messages() (PHP today only exposes the callback-based consumeMessages(callback, limit)).

Status

The PHP SDK is currently experimental. This issue stays open until the boxes above are checked or explicitly de-scoped.