fix #97 Make process_execution.context nullable, as in the mapping

[njoubert-cleverage]

Description

Fixes #97 (third column, found after #98 was merged).

process_execution.context is created as JSON NOT NULL by Version20241007152613, while ProcessExecution::$context is mapped as nullable: true. As Version20261005120000 (added by #98) has never been released, it is completed instead of adding a new migration:

	MySQL / MariaDB	PostgreSQL
up (added)	ALTER TABLE process_execution MODIFY context JSON DEFAULT NULL	ALTER TABLE process_execution ALTER context DROP NOT NULL
down (added)	ALTER TABLE process_execution MODIFY context JSON NOT NULL	ALTER TABLE process_execution ALTER context SET NOT NULL

The CHANGELOG entry of #97 describes the whole migration, as a single change.

No failure today: ProcessExecution always sets at least [], this only aligns the schema on the mapping.

Checked:

• process-bundle-demo, MySQL 9.1: down / up of Version20261005120000 (3 statements): log_record.process_execution_id NOT NULL and process_execution.context nullable, both no longer reported by doctrine:schema:update --dump-sql; a process still runs
• PostgreSQL 16 (throwaway container): fresh database, migrations OK, both columns as in the mapping, schema diff: only the cosmetic (DC2Type:...) comments; down / up OK
• bundle (Symfony 8.1, PHP 8.5): 257 tests OK. PHPStan, PHP-CS-Fixer, Rector OK

Note: a database that already executed Version20261005120000 (e.g. a development environment on main) does not get the context change: run doctrine:migrations:execute --down then --up on it.

Requirements

• Documentation updates
  • Reference
  • Changelog
• Unit tests

Breaking changes

None.

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