feat: search_tool (#10657)

**Why We Did This**
- The goal is to reduce MCP tool context pollution by not exposing the
full MCP tool list up front
- It forces an explicit discovery step (`search_tool_bm25`) so the model
narrows tool scope before making MCP calls, which helps relevance and
lowers prompt/tool clutter.

**What It Changed**
- Added a new experimental feature flag `search_tool` in
`core/src/features.rs:90` and `core/src/features.rs:430`.
- Added config/schema support for that flag in
`core/config.schema.json:214` and `core/config.schema.json:1235`.
- Added BM25 dependency (`bm25`) in `Cargo.toml:129` and
`core/Cargo.toml:23`.
- Added new tool handler `search_tool_bm25` in
`core/src/tools/handlers/search_tool_bm25.rs:18`.
- Registered the handler and tool spec in
`core/src/tools/handlers/mod.rs:11` and `core/src/tools/spec.rs:780` and
`core/src/tools/spec.rs:1344`.
- Extended `ToolsConfig` to carry `search_tool` enablement in
`core/src/tools/spec.rs:32` and `core/src/tools/spec.rs:56`.
- Injected dedicated developer instructions for tool-discovery workflow
in `core/src/codex.rs:483` and `core/src/codex.rs:1976`, using
`core/templates/search_tool/developer_instructions.md:1`.
- Added session state to store one-shot selected MCP tools in
`core/src/state/session.rs:27` and `core/src/state/session.rs:131`.
- Added filtering so when feature is enabled, only selected MCP tools
are exposed on the next request (then consumed) in
`core/src/codex.rs:3800` and `core/src/codex.rs:3843`.
- Added E2E suite coverage for
enablement/instructions/hide-until-search/one-turn-selection in
`core/tests/suite/search_tool.rs:72`,
`core/tests/suite/search_tool.rs:109`,
`core/tests/suite/search_tool.rs:147`, and
`core/tests/suite/search_tool.rs:218`.
- Refactored test helper utilities to support config-driven tool
collection in `core/tests/suite/tools.rs:281`.

**Net Behavioral Effect**
- With `search_tool` **off**: existing MCP behavior (tools exposed
normally).
- With `search_tool` **on**: MCP tools start hidden, model must call
`search_tool_bm25`, and only returned `selected_tools` are available for
the next model call.

codex-rs/Cargo.lock

@@ -922,6 +922,20 @@ dependencies = [
  "piper",
 ]
 
+[[package]]
+name = "bm25"
+version = "2.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1cbd8ffdfb7b4c2ff038726178a780a94f90525ed0ad264c0afaa75dd8c18a64"
+dependencies = [
+ "cached",
+ "deunicode",
+ "fxhash",
+ "rust-stemmers",
+ "stop-words",
+ "unicode-segmentation",
+]
+
 [[package]]
 name = "borsh"
 version = "1.6.0"
@@ -1000,6 +1014,39 @@ dependencies = [
  "pkg-config",
 ]
 
+[[package]]
+name = "cached"
+version = "0.56.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "801927ee168e17809ab8901d9f01f700cd7d8d6a6527997fee44e4b0327a253c"
+dependencies = [
+ "ahash",
+ "cached_proc_macro",
+ "cached_proc_macro_types",
+ "hashbrown 0.15.5",
+ "once_cell",
+ "thiserror 2.0.18",
+ "web-time",
+]
+
+[[package]]
+name = "cached_proc_macro"
+version = "0.25.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9225bdcf4e4a9a4c08bf16607908eb2fbf746828d5e0b5e019726dbf6571f201"
+dependencies = [
+ "darling 0.20.11",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.114",
+]
+
+[[package]]
+name = "cached_proc_macro_types"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ade8366b8bd5ba243f0a58f036cc0ca8a2f069cff1a2351ef1cac6b083e16fc0"
+
 [[package]]
 name = "cassowary"
 version = "0.3.0"
@@ -1555,6 +1602,7 @@ dependencies = [
  "async-channel",
  "async-trait",
  "base64 0.22.1",
+ "bm25",
  "chardetng",
  "chrono",
  "clap",
@@ -2704,6 +2752,16 @@ dependencies = [
  "syn 2.0.114",
 ]
 
+[[package]]
+name = "darling"
+version = "0.20.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc7f46116c46ff9ab3eb1597a45688b6715c6e628b5c133e288e709a29bcb4ee"
+dependencies = [
+ "darling_core 0.20.11",
+ "darling_macro 0.20.11",
+]
+
 [[package]]
 name = "darling"
 version = "0.21.3"
@@ -2724,6 +2782,20 @@ dependencies = [
  "darling_macro 0.23.0",
 ]
 
+[[package]]
+name = "darling_core"
+version = "0.20.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0d00b9596d185e565c2207a0b01f8bd1a135483d02d9b7b0a54b11da8d53412e"
+dependencies = [
+ "fnv",
+ "ident_case",
+ "proc-macro2",
+ "quote",
+ "strsim 0.11.1",
+ "syn 2.0.114",
+]
+
 [[package]]
 name = "darling_core"
 version = "0.21.3"
@@ -2751,6 +2823,17 @@ dependencies = [
  "syn 2.0.114",
 ]
 
+[[package]]
+name = "darling_macro"
+version = "0.20.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fc34b93ccb385b40dc71c6fceac4b2ad23662c7eeb248cf10d529b7e055b6ead"
+dependencies = [
+ "darling_core 0.20.11",
+ "quote",
+ "syn 2.0.114",
+]
+
 [[package]]
 name = "darling_macro"
 version = "0.21.3"
@@ -2955,6 +3038,12 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "deunicode"
+version = "1.6.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "abd57806937c9cc163efc8ea3910e00a62e2aeb0b8119f1793a978088f8f6b04"
+
 [[package]]
 name = "diff"
 version = "0.1.13"
@@ -7223,6 +7312,16 @@ dependencies = [
  "walkdir",
 ]
 
+[[package]]
+name = "rust-stemmers"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e46a2036019fdb888131db7a4c847a1063a7493f971ed94ea82c67eada63ca54"
+dependencies = [
+ "serde",
+ "serde_derive",
+]
+
 [[package]]
 name = "rustc-demangle"
 version = "0.1.27"
@@ -8456,6 +8555,15 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a2eb9349b6444b326872e140eb1cf5e7c522154d69e7a0ffb0fb81c06b37543f"
 
+[[package]]
+name = "stop-words"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "645a3d441ccf4bf47f2e4b7681461986681a6eeea9937d4c3bc9febd61d17c71"
+dependencies = [
+ "serde_json",
+]
+
 [[package]]
 name = "streaming-iterator"
 version = "0.1.9"