Fix child task compute provider inheritance

[daniel-lxs]

What changed

• Inherit the authenticated source task run's compute provider for run-token child launches when the request does not specify one.
• Preserve explicit compute-provider overrides and the existing deployment-default behavior for ordinary user-token launches.
• Cover Docker and Modal inheritance plus explicit override behavior.

Root cause

Agent-launched child tasks omit computeProvider, so fresh-task routing selected the deployment default instead of the provider that ran the parent. Verification and delegated work could therefore execute on a different backend from the environment they were intended to reproduce.

Impact

Child tasks now stay on the parent's compute backend by default, while callers that explicitly choose a provider retain that choice.

Validation

• Targeted API test: 17 passed
• @roomote/api TypeScript check
• Changed-file formatting and ESLint
• pnpm lint:fast
• pnpm check-types:fast
• pnpm knip

[roomote-community]

No code issues found. See task

_{Reviewed 396dae4}