Design: Remove cebe/php-openapi dependency (issue #418)

Problem

cebe/php-openapi: ^1.8 is listed in the require section of composer.json, meaning it is installed in production environments as a runtime dependency. However, the package is entirely unused:

• No PHP file in src/ or tools/ imports any class from the cebe\openapi namespace.
• All OpenAPI schema parsing in src/OpenApi/Domain/ uses native json_decode().
• SchemaBuilder command simply fetches the raw JSON payload from the Bitrix24 REST API and writes it to a file.
• No configuration file (deptrac.yaml, phpstan.dist.neon, .php-cs-fixer.php, rector.php) references the package.

Decision

Remove cebe/php-openapi entirely from composer.json (not move to require-dev).

Rationale: the package is dead code at the dependency level. Moving it to require-dev would keep installing an unused package for all contributors. Removal is the cleanest outcome and aligns with YAGNI — if cebe-based tooling is needed in the future, it can be added back then.

Scope of change

No PHP source files, no test files, no configuration files require any modification.

Verification

make lint-allowed-licenses
make lint-cs-fixer
make lint-phpstan
make lint-rector
make lint-deptrac
make test-unit