Remove unavailable MCP placeholder tool backfill (#22439)

## 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`

codex-rs/features/src/lib.rs

@@ -162,7 +162,7 @@ pub enum Feature {
     ToolSearch,
     /// Always defer MCP tools behind tool_search instead of exposing small sets directly.
     ToolSearchAlwaysDeferMcpTools,
-    /// Expose placeholder tools for unavailable historical tool calls.
+    /// Removed compatibility flag for the deleted unavailable-tool placeholder backfill.
     UnavailableDummyTools,
     /// Enable discoverable tool suggestions for apps.
     ToolSuggest,
@@ -958,8 +958,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::UnavailableDummyTools,
         key: "unavailable_dummy_tools",
-        stage: Stage::Stable,
-        default_enabled: true,
+        stage: Stage::Removed,
+        default_enabled: false,
     },
     FeatureSpec {
         id: Feature::ToolSuggest,