test(gateway): add webhook toolset regression coverage

[dlkakbs]

Summary

Adds explicit regression coverage for the webhook platform/toolset wiring that previously caused KeyError failures on webhook-triggered agent runs.

This PR verifies that:

• hermes-webhook is defined and validates as a toolset
• the webhook platform resolves to default_toolset="hermes-webhook"
• hermes_cli.tools_config.PLATFORMS exports the same mapping
• hermes-gateway still includes hermes-webhook

Context

This is a focused test-only follow-up to earlier webhook platform registration work.

Related history:

• fix(gateway): register webhook platform and toolset to fix KeyError on agent runs  #3775 originally proposed the runtime fix plus a dedicated regression test
• during review on fix(gateway): register webhook platform and toolset to fix KeyError on agent runs  #3775, the runtime fix was noted as already having landed on main
• that runtime wiring was merged via fix: webhook platform support — skip home channel prompt, disable tool progress (salvage #4363) #4660
• fix: webhook platform support — skip home channel prompt, disable tool progress (salvage #4363) #4660 itself was a salvage/cherry-pick follow-up from fix: add webhook platform support and skip home channel prompt #4363

What landed on main from that chain was the runtime/config wiring:

• webhook platform registration
• hermes-webhook toolset registration
• inclusion in hermes-gateway

What did not land was the dedicated regression coverage from #3775.

This PR adds that missing explicit test coverage only, without changing runtime behavior.

Why this is still useful

main already has broader platform/toolset consistency coverage, but this PR adds a webhook-specific regression test for the exact wiring that was previously missing and that motivated #3775.

That makes the original failure mode easier to catch directly if webhook registration regresses again.

[teknium1]

Thanks for the follow-up — closing as redundant.

The exact wiring this file pins is already covered generically by TestPlatformToolsetConsistency in tests/hermes_cli/test_tools_config.py (around L558):

• test_all_platforms_have_toolset_definitions asserts every entry in tools_config.PLATFORMS (including webhook) has its default_toolset defined in TOOLSETS — would fail if hermes-webhook got unwired.
• test_gateway_toolset_includes_all_messaging_platforms asserts every messaging platform's toolset is in hermes-gateway["includes"] — webhook is in that sweep.

So the original failure mode (webhook platform registered without its toolset, or missing from the gateway bundle) already fails the existing generic suite. Adding a webhook-named duplicate doesn't give us additional regression coverage — it just doubles the assertion surface for one platform while every other platform rides on the generic check.

Appreciate the diligence on the #3775 → #4363 → #4660 chain — the runtime wiring is landed and guarded.