fix: honor IGNORE_ON_ENTER for tools inside a Toolset

[longcw]

Closes #6354

ToolFlag.IGNORE_ON_ENTER was only honored for bare FunctionTool/RawFunctionTool instances in the on_enter reply filter — tools nested in a Toolset (e.g. EndCallTool) passed through unfiltered, so the model could fire a speculative end_call during the greeting turn (with delete_room=True, an instant hangup at pickup).

The filter now flattens toolsets and hides flagged tools via ToolContext._exclude, which drops them from the callable/LLM-visible set while keeping the owning toolset in place so executor routing and lifecycle are unaffected. Rather than threading the ignored set through the reply tasks, _on_enter_ignored_tools reads the on_enter contextvar directly; the reply task and its tool-response follow-ups inherit it through asyncio.create_task.

The realtime path is covered too: it only overrides its session tools when generate_reply is given tools=, so inside on_enter it now falls back to the agent's full set and hides flagged tools even without an explicit tools=.

EndCallTool gains an ignore_on_enter parameter (defaults to False, preserving current behavior) to opt into hiding end_call during the greeting.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.