fix(generators): execute task generators atomically

[tobiasraabe]

Summary

• stop task generators after the generator hook has handled them so their bodies execute once
• collect every generated child report and surface all child collection failures
• commit generated tasks, the rebuilt DAG, and the rebuilt scheduler as one in-memory batch
• process a failed generator like a normal task failure so descendants skip and unrelated tasks continue
• preserve the identity of the session task list for plugin compatibility

Root cause

pytask_execute_task is a first-result hook, but the generator implementation returned None after executing a generator. Pluggy therefore continued to the default executor and called the generator body a second time. The implementation also committed successful generated siblings before checking failed collection reports and only retained the final report.

Impact

A generator now executes once. Generated child collection is atomic: all reports are retained for diagnostics, but no generated child or rebuilt DAG is committed when any child fails. The generator receives one ordinary failed execution report, its descendants are skipped, and unrelated tasks can continue.

This PR changes only core pytask. pytask-parallel requires its separately coordinated controller-side generator handling before it can support this contract.