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/core/src/session/tests.rs

@@ -542,7 +542,6 @@ fn test_tool_runtime(session: Arc<Session>, turn_context: Arc<TurnContext>) -> T
         crate::tools::router::ToolRouterParams {
             mcp_tools: None,
             deferred_mcp_tools: None,
-            unavailable_called_tools: Vec::new(),
             discoverable_tools: None,
             extension_tool_bundles: Vec::new(),
             dynamic_tools: turn_context.dynamic_tools.as_slice(),
@@ -8607,7 +8606,6 @@ async fn fatal_tool_error_stops_turn_and_reports_error() {
         crate::tools::router::ToolRouterParams {
             deferred_mcp_tools,
             mcp_tools: Some(tools),
-            unavailable_called_tools: Vec::new(),
             discoverable_tools: None,
             extension_tool_bundles: Vec::new(),
             dynamic_tools: turn_context.dynamic_tools.as_slice(),

codex-rs/core/src/session/turn.rs

@@ -58,7 +58,6 @@ use crate::tools::router::ToolRouterParams;
 use crate::tools::router::extension_tool_bundles;
 use crate::turn_diff_tracker::TurnDiffTracker;
 use crate::turn_timing::record_turn_ttft_metric;
-use crate::unavailable_tool::collect_unavailable_called_tools;
 use crate::util::backoff;
 use crate::util::error_or_panic;
 use codex_analytics::AppInvocation;
@@ -1245,27 +1244,11 @@ pub(crate) async fn built_tools(
     );
     let mcp_tools = has_mcp_servers.then_some(mcp_tool_exposure.direct_tools);
     let deferred_mcp_tools = mcp_tool_exposure.deferred_tools;
-    let unavailable_called_tools = if turn_context
-        .config
-        .features
-        .enabled(Feature::UnavailableDummyTools)
-    {
-        let exposed_tool_names = mcp_tools
-            .iter()
-            .chain(deferred_mcp_tools.iter())
-            .flat_map(|tools| tools.iter().map(codex_mcp::ToolInfo::canonical_tool_name))
-            .collect::<HashSet<_>>();
-        collect_unavailable_called_tools(input, &exposed_tool_names)
-    } else {
-        Vec::new()
-    };
-
     Ok(Arc::new(ToolRouter::from_config(
         &turn_context.tools_config,
         ToolRouterParams {
             mcp_tools,
             deferred_mcp_tools,
-            unavailable_called_tools,
             discoverable_tools,
             extension_tool_bundles: extension_tool_bundles(sess),
             dynamic_tools: turn_context.dynamic_tools.as_slice(),