fix(kagent-adk): surface the sub-agent's ask_user question in remote_hitl_hint

[vramahandry]

Summary

remote_hitl_hint() in the Python kagent-adk runtime builds the hint text shown to a human when a sub-agent's own HITL pause bubbles up to the parent agent. Unlike the Go runtime's RemoteHitlHint(), it only ever lists the paused tool's name (e.g. ask_user) — never the actual question — even though AskUserRequest.questions (set directly on the request in build_hitl_status_message, whether or not nested is populated) has the real content right there.

• Before: "Remote agent 'github_agent' requires approval for tool(s): ask_user"
• After: "Remote agent 'github_agent' asks: What is the GitHub owner/org for the repo?"

Real tool-approval hints (the non-ask_user case) are unaffected.

This is the Python-runtime counterpart to #2475, which fixed the same class of bug in go/adk/pkg/a2a/hitl.go. The Go and Python runtimes maintain independent implementations of this HITL hint logic, and the Python side never had question-surfacing added — so a parent agent built on kagent-adk still drops the question today even after #2475 merges.

Unlike the Go fix, there's no []any vs []map[string]any] JSON-decode subtlety to worry about here: Pydantic's HitlTool.args: dict[str, Any] keeps nested question dicts intact, so reading AskUserRequest.questions directly is straightforward in both the direct and nested case.

Fixes #2473

Changes

• python/packages/kagent-adk/src/kagent/adk/_hitl.py: remote_hitl_hint() now checks AskUserRequest.questions first and returns "Remote agent '{name}' asks: {question}" when present, falling back to the existing tool-name-only wording otherwise.
• python/packages/kagent-adk/tests/unittests/test_hitl.py: adds test_remote_hitl_hint_tool_approval, test_remote_hitl_hint_ask_user, and test_remote_hitl_hint_ask_user_nested (the last covering a two-level nested ask_user pause).

Test plan

• uv run pytest packages/kagent-adk/tests/unittests/test_hitl.py — 18 passed (3 new)
• uv run ruff format --diff / uv run ruff check — clean
• Existing tests unaffected — no changes to tool-approval hint wording