fix(app-server): align configWarning notification nullability

[andrei-hasna]

Summary

Aligns the v2 configWarning notification wire shape with the app-server explicit-null payload contract. ConfigWarningNotification.path and .range were using #[serde(skip_serializing_if = "Option::is_none")] + #[ts(optional)], so they were omitted when absent — contrary to the repo rule that active v2 notification payload fields serialize nullable fields as present null, not omission.

Changes

• app-server-protocol/src/protocol/v2/config.rs: Remove skip_serializing_if + #[ts(optional)] from path and range so serde always emits them. Add a RequiredNullable<T> schemars wrapper (#[schemars(with = "...", required)]) so both fields are represented as present-nullable and listed in the schema required set while remaining nullable.
• app-server/src/outgoing_message.rs: verify_config_warning_notification_serialization now asserts path: null and range: null for the no-path/no-range case.
• app-server/README.md: configWarning documented as { summary, details, path, range } with nullable fields present as null (no longer details?, path?, range?).
• Regenerated fixtures (just write-app-server-schema): JSON schema (ServerNotification.json, codex_app_server_protocol.schemas.json, codex_app_server_protocol.v2.schemas.json, v2/ConfigWarningNotification.json) now list path/range in required as present-nullable; TypeScript fixture shows path: string | null and range: TextRange | null.

Scope is limited to ConfigWarningNotification; no unrelated warning/config/thread-item/skills cleanup.

Validation

• just write-app-server-schema — no diff (fixtures already regenerated)
• just test-fast -p codex-app-server-protocol — 256 passed
• just test-fast -p codex-app-server verify_config_warning_notification_serialization — 1 passed
• just fmt — clean
• just fix -p codex-app-server-protocol / just fix -p codex-app-server — clippy clean
• Independent adversarial verification: CONFIRMED (schema keeps path/range required-present-nullable, no out-of-scope changes)

Task: f99c6654-c6a9-4999-9579-3427e35fd5aa

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