docs(sdk): document background delegation

[KirschBluteX]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• Document the TaskToolSet background lifecycle added by OpenHands/software-agent-sdk#4503.
• Show the blocking default and explicit run_in_background=True opt-in.
• Document task_output, task_stop, task states, cooperative cancellation, bounded close behavior, and exactly-once metrics settlement.
• Clarify that task IDs are process-local and are not restored after a process restart; active resume is rejected.

Validation

• git diff --check passed.
• The documented imports and lifecycle match the SDK implementation and its 165 focused/related tests.
• Full-checkout Mint and desktop/mobile render checks were run previously. The final sparse checkout reports baseline missing-file/navigation warnings for files outside this PR; those files are unchanged here.

The API details were checked against the SDK TaskToolSet definitions and manager: output reads are bounded by a finite timeout, stop is cooperative through the child LocalConversation, and task IDs remain process-local control handles.