## Why
`UnavailableDummyTools` kept synthetic placeholder tools alive for
historical tool calls whose backing MCP tool was no longer available.
That path adds stale model-visible tool specs and special routing at the
point where unavailable MCP calls should use ordinary current-tool
handling. This removes the runtime backfill instead of preserving a
second compatibility lane.
## Is it safe to remove?
The unavailable tools were added in #17853 after a CS issue when a
previously-called MCP tool failed to load and was omitted from the CS
spec. Now that we have tool search, I think this is resolved:
- API merges tools from previous TST output into effective tool set so
theyre always in CS spec
- if an MCP tool surfaced by TST later becomes unavailable, the model
can still call it and it will just return model-visible error
- both TST output and function call output are dropped on compaction so
model will not remember old calls to MCP post compaction
## What changed
- Delete unavailable-tool collection, placeholder handler, router/spec
plumbing, and obsolete placeholder coverage.
- Keep `features.unavailable_dummy_tools` as a removed no-op feature
tombstone so existing configs still parse cleanly.
- Add an integration-style `tool_search` regression test showing that a
deferred MCP tool surfaced through `tool_search` still routes through
MCP and returns a model-visible tool-call error rather than `unsupported
call`.
## Verification
- `cargo test -p codex-core tool_search`
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