[app-server] make file_path for config optional (#7560)

When we are writing to config using `config/value/write` or
`config/batchWrite`, it always require a `config/read` before it right
now in order to get the correct file path to write to. make this
optional so we read from the default user config file if this is not
passed in.

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

@@ -209,6 +209,8 @@ pub struct OverriddenMetadata {
 pub struct ConfigWriteResponse {
     pub status: WriteStatus,
     pub version: String,
+    /// Canonical path to the config file that was written.
+    pub file_path: String,
     pub overridden_metadata: Option<OverriddenMetadata>,
 }
 
@@ -245,10 +247,11 @@ pub struct ConfigReadResponse {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct ConfigValueWriteParams {
-    pub file_path: String,
     pub key_path: String,
     pub value: JsonValue,
     pub merge_strategy: MergeStrategy,
+    /// Path to the config file to write; defaults to the user's `config.toml` when omitted.
+    pub file_path: Option<String>,
     pub expected_version: Option<String>,
 }
 
@@ -256,8 +259,9 @@ pub struct ConfigValueWriteParams {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct ConfigBatchWriteParams {
-    pub file_path: String,
     pub edits: Vec<ConfigEdit>,
+    /// Path to the config file to write; defaults to the user's `config.toml` when omitted.
+    pub file_path: Option<String>,
     pub expected_version: Option<String>,
 }