Remove unavailable MCP placeholder tool backfill

[sayan-oai]

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