feat: add NOOP workflow task builder

[abhinavrastogi2627]

Fixes #430.

The server supports the NOOP system task, but Python workflows cannot construct it through the SDK. Add TaskType.NOOP and NoopTask(task_ref_name) using the existing task builder interface, with an example of an explicit no-op SWITCH default branch.

Tests verify the wire payload both as a standalone task and nested in a SWITCH, including the reference name and empty input parameters. No generated HTTP models or existing task behavior are changed.

Validation:

• Unit suite: 2,673 passed, 1 skipped.
• Workflow, backward-compatibility, and serialization suites: 1,086 passed, plus 671 subtests.
• Documentation example and new-module lint pass; git diff --check passes.
• The existing TaskType.LLM_INDEX_DOCUMENT alias triggers Ruff PIE796 on both unchanged main and this branch.
• The combined five-suite command encounters two worker-registry test failures and 48 integration setup errors without a live server; the exact same 50 failing/error test IDs reproduce on unchanged main (2248984). Running the unit suite independently passes on both revisions.

No live Conductor execution was performed. The builder's wire type was cross-checked with the server's TaskType.NOOP and Noop system-task implementation.