Fix examples, model defaults and topic NUL bytes from SDK review #2052

Tracks PR #2050.

Plan

Goal: Fix the review findings on the SDK major PRs at their source in the generator: examples that don't compile in PHP, Ruby, Dart and Flutter, wrong model defaults in Python and Dart/Flutter, and Topic helpers that accept NUL bytes.

What changes (all in #2050):

• Examples: src/SDK/Language/PHP.php (object arrays render as PHP arrays, escaped quotes), src/SDK/Language/Ruby.php (nil for empty optional numbers, single-quoted escaped strings), src/SDK/Language/Dart.php plus templates/dart/docs/example.md.twig and templates/flutter/docs/example.md.twig (text responses typed String, escaped enum member names like xnew).
• Model defaults: src/SDK/Language/Python.php (a map field's {} default stays {}), templates/dart/lib/src/models/model.dart.twig (required nested models fall back to an empty map).
• Topic: every language's Topic template (templates/*/…/Topic* / topic.*.twig) rejects U+0000.
• Tests: new preset/settings and preset/profile mock endpoints in mock-server/app/http.php and tests/resources/spec-openapi3.json, new e2e expectations in tests/e2e/Base.php and the per-language tests, and the applyDefaults count in tests/generation/GenerationTest.php.

Remaining steps: None beyond the linked PR; once released, the SDK major PRs pick up the regenerated output.

Done when:

• fix: examples, model defaults and topic NUL bytes from SDK review #2050 is reviewed and merged
• e2e passes for the affected languages (Python, Dart/Flutter, Ruby, PHP and the Topic cases)
• The fix ships in an sdk-generator release