[Enhancement] Expose omitted tool identifiers when workflow tool catalogue falls back or is truncated

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

When a workflow has more than 80 installed tools, the workflow generator uses tool routing to select the tools rendered into the planner prompt. If the router returns unmatched queries or raises an exception, the code falls back to select_legacy_fallback_tools() and keeps only up to 80 entries.

The current fallback logs are useful, but they mainly expose selected counts such as total, candidates, and in the unmatched-query branch pinned. They do not expose which tools were omitted from the prompt. Also, select_legacy_fallback_tools() currently returns only the selected list, so callers cannot recover the omitted entries.

This makes production troubleshooting difficult when an installed tool appears to be unavailable to the agent. It is hard to distinguish between “the tool was not installed”, “the router did not select it”, and “the tool was silently removed by fallback truncation”.

2. Additional context or comments

I would like to propose a small observability enhancement without changing tool-selection semantics:

1. Return a structured fallback selection result containing entries, omitted_entries, pinned_count, and limit.
2. Keep select_legacy_fallback_tools() compatible, either through a new function or a compatibility wrapper.
3. In the fallback warning, add omitted, limit, overflow, and a capped preview such as omitted_tools, limited to 20 provider/tool identifiers to avoid log bloat.
4. Preserve the existing fallback order, pinned-tool behavior, and rendered prompt output.
5. Add tests for the non-truncated case, exactly 80 tools, 81 tools, pinned overflow, and successful router selection.

I checked the current main branch and found that the fallback path still does not expose omitted tool identities.

3. Can you help us with this feature?

• I am interested in contributing to this feature.