refactor(mcp): preserve TOML formatting when syncing to Codex

Switch from `toml` to `toml_edit` crate for incremental TOML editing,
preserving user-written comments, whitespace, and key ordering in
Codex config.toml.

Changes:
- Add toml_edit 0.22 dependency for preserving-style TOML editing
- Refactor sync_enabled_to_codex() to use toml_edit::DocumentMut API
- Implement smart style detection: inherit existing mcp.servers or
  mcp_servers style, with fallback to sensible defaults
- Add deduplication logic to prevent both styles coexisting
- Add tests for format preservation and style inheritance
- Fix unused_mut and nonminimal_bool compiler warnings
- Apply cargo fmt to all modified files

Benefits:
- User comments and formatting are no longer lost during sync
- Respects user's preferred TOML structure (nested vs toplevel)
- Non-MCP fields in config.toml remain untouched
- Minimal surprise principle: only modifies necessary sections

Testing:
- All 47 tests passing (unit + integration)
- Clippy clean (0 warnings, 0 errors)
- Release build successful
- New tests verify comment preservation and style detection

src-tauri/Cargo.lock

@@ -588,6 +588,7 @@ dependencies = [
  "thiserror 1.0.69",
  "tokio",
  "toml 0.8.2",
+ "toml_edit 0.22.27",
 ]
 
 [[package]]
@@ -1573,7 +1574,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0bb0228f477c0900c880fd78c8759b95c7636dbd7842707f49e132378aa2acdc"
 dependencies = [
  "heck 0.4.1",
- "proc-macro-crate 2.0.2",
+ "proc-macro-crate 2.0.0",
  "proc-macro-error",
  "proc-macro2",
  "quote",
@@ -3110,11 +3111,10 @@ dependencies = [
 
 [[package]]
 name = "proc-macro-crate"
-version = "2.0.2"
+version = "2.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b00f26d3400549137f92511a46ac1cd8ce37cb5598a96d382381458b992a5d24"
+checksum = "7e8366a6159044a37876a2b9817124296703c586a5c92e2c53751fa06d8d43e8"
 dependencies = [
- "toml_datetime 0.6.3",
  "toml_edit 0.20.2",
 ]
 
@@ -4876,7 +4876,7 @@ checksum = "185d8ab0dfbb35cf1399a6344d8484209c088f75f8f68230da55d48d95d43e3d"
 dependencies = [
  "serde",
  "serde_spanned 0.6.9",
- "toml_datetime 0.6.3",
+ "toml_datetime 0.6.11",
  "toml_edit 0.20.2",
 ]
 
@@ -4897,9 +4897,9 @@ dependencies = [
 
 [[package]]
 name = "toml_datetime"
-version = "0.6.3"
+version = "0.6.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7cda73e2f1397b1262d6dfdcef8aafae14d1de7748d66822d3bfeeb6d03e5e4b"
+checksum = "22cddaf88f4fbc13c51aebbf5f8eceb5c7c5a9da2ac40a13519eb5b0a0e8f11c"
 dependencies = [
  "serde",
 ]
@@ -4920,7 +4920,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1b5bb770da30e5cbfde35a2d7b9b8a2c4b8ef89548a7a6aeab5c9a576e3e7421"
 dependencies = [
  "indexmap 2.11.4",
- "toml_datetime 0.6.3",
+ "toml_datetime 0.6.11",
  "winnow 0.5.40",
 ]
 
@@ -4933,10 +4933,22 @@ dependencies = [
  "indexmap 2.11.4",
  "serde",
  "serde_spanned 0.6.9",
- "toml_datetime 0.6.3",
+ "toml_datetime 0.6.11",
  "winnow 0.5.40",
 ]
 
+[[package]]
+name = "toml_edit"
+version = "0.22.27"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41fe8c660ae4257887cf66394862d21dbca4a6ddd26f04a3560410406a2f819a"
+dependencies = [
+ "indexmap 2.11.4",
+ "toml_datetime 0.6.11",
+ "toml_write",
+ "winnow 0.7.13",
+]
+
 [[package]]
 name = "toml_edit"
 version = "0.23.6"
@@ -4958,6 +4970,12 @@ dependencies = [
  "winnow 0.7.13",
 ]
 
+[[package]]
+name = "toml_write"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d99f8c9a7727884afe522e9bd5edbfc91a3312b36a77b5fb8926e4c31a41801"
+
 [[package]]
 name = "toml_writer"
 version = "1.0.3"