[codex] Add remote thread store implementation (#17826)

- Add a "remote" thread store implementation
- Implement the remote thread store as a thin wrapper that makes grpc
calls to a configurable service endpoint
- Implement only the thread/list method to start
- Encode the grpc method/param shape as protobufs in the remote
implementation

A wart: the proto generation script is an "example" binary target. This
is an example target only because Cargo lets examples use
dev-dependencies, which keeps tonic-prost-build out of the normal
codex-thread-store dependency surface. A regular bin would either need
to add proto generation deps as normal runtime deps, or use a
feature-gated optional dep, which this repo’s manifest checks explicitly
reject.

codex-rs/Cargo.lock

@@ -2886,11 +2886,16 @@ dependencies = [
  "codex-rollout",
  "codex-state",
  "pretty_assertions",
+ "prost 0.14.3",
  "serde",
  "serde_json",
  "tempfile",
  "thiserror 2.0.18",
  "tokio",
+ "tokio-stream",
+ "tonic",
+ "tonic-prost",
+ "tonic-prost-build",
  "uuid",
 ]
 
@@ -7561,7 +7566,7 @@ dependencies = [
  "heck 0.4.1",
  "itertools 0.11.0",
  "prost 0.12.6",
- "prost-types",
+ "prost-types 0.12.6",
 ]
 
 [[package]]
@@ -7575,7 +7580,7 @@ dependencies = [
  "pbjson",
  "pbjson-build",
  "prost 0.12.6",
- "prost-build",
+ "prost-build 0.12.6",
  "serde",
 ]
 
@@ -7999,7 +8004,26 @@ dependencies = [
  "petgraph 0.6.5",
  "prettyplease",
  "prost 0.12.6",
- "prost-types",
+ "prost-types 0.12.6",
+ "regex",
+ "syn 2.0.114",
+ "tempfile",
+]
+
+[[package]]
+name = "prost-build"
+version = "0.14.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "343d3bd7056eda839b03204e68deff7d1b13aba7af2b2fd16890697274262ee7"
+dependencies = [
+ "heck 0.5.0",
+ "itertools 0.14.0",
+ "log",
+ "multimap",
+ "petgraph 0.8.3",
+ "prettyplease",
+ "prost 0.14.3",
+ "prost-types 0.14.3",
  "regex",
  "syn 2.0.114",
  "tempfile",
@@ -8040,6 +8064,15 @@ dependencies = [
  "prost 0.12.6",
 ]
 
+[[package]]
+name = "prost-types"
+version = "0.14.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8991c4cbdb8bc5b11f0b074ffe286c30e523de90fee5ba8132f1399f23cb3dd7"
+dependencies = [
+ "prost 0.14.3",
+]
+
 [[package]]
 name = "psl"
 version = "2.1.184"
@@ -10948,8 +10981,10 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a286e33f82f8a1ee2df63f4fa35c0becf4a85a0cb03091a15fd7bf0b402dc94a"
 dependencies = [
  "async-trait",
+ "axum",
  "base64 0.22.1",
  "bytes",
+ "h2",
  "http 1.4.0",
  "http-body",
  "http-body-util",
@@ -10959,6 +10994,7 @@ dependencies = [
  "percent-encoding",
  "pin-project",
  "rustls-native-certs",
+ "socket2 0.6.2",
  "sync_wrapper",
  "tokio",
  "tokio-rustls",
@@ -10969,6 +11005,18 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "tonic-build"
+version = "0.14.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "27aac809edf60b741e2d7db6367214d078856b8a5bff0087e94ff330fb97b6fc"
+dependencies = [
+ "prettyplease",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.114",
+]
+
 [[package]]
 name = "tonic-prost"
 version = "0.14.3"
@@ -10980,6 +11028,22 @@ dependencies = [
  "tonic",
 ]
 
+[[package]]
+name = "tonic-prost-build"
+version = "0.14.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a4556786613791cfef4ed134aa670b61a85cfcacf71543ef33e8d801abae988f"
+dependencies = [
+ "prettyplease",
+ "proc-macro2",
+ "prost-build 0.14.3",
+ "prost-types 0.14.3",
+ "quote",
+ "syn 2.0.114",
+ "tempfile",
+ "tonic-build",
+]
+
 [[package]]
 name = "tower"
 version = "0.5.3"