feat(rust-only): better tolerate malformed optional fields in deserialization (#1006)

* feat(rust-only): better tolerate malformed optional fields in deserialization

Use `serde_with` helpers to default invalid optional values and
skip bad vector entries across protocol types.

Sometimes agents or clients send something not quite right. If it is an
optional field, we can fallback to best-effort and retain as much
information as possible without losing everything.

* Apply `skip_serializing_none` across protocol structs so optional
fields are omitted consistently.

* Add optional tracing for skipped list entries

Introduce a `tracing` feature that logs `VecSkipError` drops via a
shared `SkipListener` inspector, while remaining a zero-cost no-op
when the feature is disabled.

Also add tests covering inspector callbacks for malformed entries.

* Make List Deserialization Fall Back to Defaults

Wrap list deserializers with `DefaultOnError` so invalid outer
shapes default instead of failing, while still skipping invalid
elements inside valid arrays.

Author: benbrandt

Cargo.lock

@@ -45,6 +45,11 @@ unstable_session_usage = []
 unstable_message_id = []
 unstable_boolean_config = []
 
+# Emit `tracing::warn!` events when `VecSkipError` drops a malformed list
+# entry during deserialization. When disabled (the default), the inspector
+# hook compiles down to serde_with's built-in no-op and has zero runtime cost.
+tracing = ["dep:tracing"]
+
 [[bin]]
 name = "generate"
 path = "src/bin/generate.rs"
@@ -55,7 +60,9 @@ derive_more = { version = "2", features = ["from", "display"] }
 schemars = { version = "1" }
 serde = { version = "1", features = ["derive", "rc"] }
 serde_json = { version = "1", features = ["raw_value"] }
+serde_with = { version = "3.18.0", features = ["json", "schemars_1"] }
 strum = { version = "0.28", features = ["derive"] }
+tracing = { version = "0.1", default-features = false, optional = true }
 
 [lints.rust]
 future_incompatible = { level = "warn", priority = -1 }

Cargo.toml

@@ -0,0 +1,5 @@
+allowed-duplicate-crates = [
+    "hashbrown",
+    "indexmap",
+    "schemars",
+]