feat: support in-repo .codex/config.toml entries as sources of config info (#8354)

- We now support `.codex/config.toml` in repo (from `cwd` up to the
first `.git` found, if any) as layers in `ConfigLayerStack`. A new
`ConfigLayerSource::Project` variant was added to support this.
- In doing this work, I realized that we were resolving relative paths
in `config.toml` after merging everything into one `toml::Value`, which
is wrong: paths should be relativized with respect to the folder
containing the `config.toml` that was deserialized. This PR introduces a
deserialize/re-serialize strategy to account for this in
`resolve_config_paths()`. (This is why `Serialize` is added to so many
types as part of this PR.)
- Added tests to verify this new behavior.



---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/8354).
* #8359
* __->__ #8354

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -240,6 +240,14 @@ pub enum ConfigLayerSource {
         file: AbsolutePathBuf,
     },
 
+    /// Path to a .codex/ folder within a project. There could be multiple of
+    /// these between `cwd` and the project/repo root.
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
+    Project {
+        dot_codex_folder: AbsolutePathBuf,
+    },
+
     /// Session-layer overrides supplied via `-c`/`--config`.
     SessionFlags,
 
@@ -247,6 +255,8 @@ pub enum ConfigLayerSource {
     /// as the last layer on top of everything else. This scheme did not quite
     /// work out as intended, but we keep this variant as a "best effort" while
     /// we phase out `managed_config.toml` in favor of `requirements.toml`.
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
     LegacyManagedConfigTomlFromFile {
         file: AbsolutePathBuf,
     },
@@ -262,6 +272,7 @@ impl ConfigLayerSource {
             ConfigLayerSource::Mdm { .. } => 0,
             ConfigLayerSource::System { .. } => 10,
             ConfigLayerSource::User { .. } => 20,
+            ConfigLayerSource::Project { .. } => 25,
             ConfigLayerSource::SessionFlags => 30,
             ConfigLayerSource::LegacyManagedConfigTomlFromFile { .. } => 40,
             ConfigLayerSource::LegacyManagedConfigTomlFromMdm => 50,