Fix 10 functional-workflow API bugs from /ultrareview pass

- bug_001: `ctx.request_info()` without an explicit `request_id` now derives
  a deterministic `auto::<index>` id from the call-counter, so HITL resume
  works correctly on the documented default path.  A uuid was regenerated on
  every replay, making resume impossible.

- bug_002: `StepWrapper.__call__` no longer deepcopies arguments on the
  cache-hit replay branch.  The copy is only performed on the live-execution
  path (for the event log) and falls back to the original mapping if deepcopy
  fails, so steps whose args aren't deepcopyable (locks, sockets, sessions)
  can still resume from checkpoint.

- bug_007: `_set_responses` now prunes each resolved `request_id` from
  `_pending_requests`, and the cache-hit branch in `request_info` does the
  same.  Previously, answered requests were re-serialized into every
  subsequent checkpoint and the final checkpoint falsely claimed pending
  requests even after the workflow completed.

- bug_008: `_compute_signature_hash` now mixes the function's `co_code` and
  `co_names` into the checkpoint signature, so changes to the workflow body
  invalidate older checkpoints even when steps are accessed via module /
  class attributes (which `_discover_step_names` can't see statically).
  `RunContext._record_observed_step` records observed step names for
  diagnostics.

- bug_010: `FunctionalWorkflow.run()` docstring corrected — says "at least
  one of message/responses/checkpoint_id" and explicitly notes `responses`
  may be combined with `checkpoint_id` (the validator already allowed this).

- bug_013: `FunctionalWorkflowAgent` now surfaces `request_info` events as
  `FunctionApprovalRequestContent` items (mirroring graph `WorkflowAgent`),
  threads `responses=` and `checkpoint_id=` through to the underlying
  workflow, and exposes `pending_requests`.  Previously `.as_agent()`
  returned empty `AgentResponse` for HITL workflows — effectively unusable.

- bug_014: `FunctionalWorkflow` now clears `_last_message`,
  `_last_step_cache`, and `_last_pending_request_ids` on clean completion.
  `run()` validates that `responses=` keys intersect the currently-pending
  request set (or raises with a clear error) instead of silently replaying
  against stale singleton state from a prior run.

- bug_015: `FunctionalWorkflow.as_agent` signature now matches graph
  `Workflow.as_agent`: accepts `name`, `description`, `context_providers`,
  and `**kwargs`.  `FunctionalWorkflowAgent` stores the overrides.

- bug_017: `RunContext.set_state` raises `ValueError` for underscore-
  prefixed keys (the framework's `_step_cache` / `_original_message` keys
  would silently clobber user state on checkpoint save and user
  underscore-prefixed state was dropped on restore).  Docstring documents
  the reserved prefix.

- merged_bug_003: Workflow function arity is validated at decoration time.
  Multiple non-ctx parameters raise `ValueError` immediately (previously
  every arg past the first was silently dropped at call time).  Passing a
  non-None `message` to a ctx-only workflow raises `ValueError` instead of
  silently discarding the message.

Test coverage: +18 regression tests covering every fix.  Full workflow
suite now 766 passed, 1 skipped, 2 xfailed; full core suite 2338 passed.

Author: moonbox3