Restore provider compatibility diagnostics as API types

Keep the G008 capability and diagnostic helpers compile-ready by restoring the public report/support/severity types that team integrations referenced after merge reconciliation.

Constraint: Final G008 verification failed on missing provider capability and diagnostic type definitions.
Confidence: high
Scope-risk: narrow
Directive: Keep provider diagnostics exported as typed API surfaces; do not replace them with ad-hoc JSON-only status fields.
Tested: cargo fmt --manifest-path rust/Cargo.toml --all -- --check; git diff --check; cargo test --manifest-path rust/Cargo.toml -p api providers:: -- --nocapture --test-threads=1; cargo test --manifest-path rust/Cargo.toml -p api --test openai_compat_integration -- --nocapture --test-threads=1
Not-tested: full workspace clippy; known unrelated runtime policy_engine struct_excessive_bools remains outside G008.

Co-authored-by: OmX <omx@oh-my-codex.dev>

rust/crates/api/src/providers/mod.rs

@@ -49,6 +49,55 @@ pub struct ModelTokenLimit {
     pub context_window_tokens: u32,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum ProviderWireProtocol {
+    AnthropicMessages,
+    OpenAiChatCompletions,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum ProviderFeatureSupport {
+    Supported,
+    Unsupported,
+    PassthroughAsTool,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
+pub struct ProviderCapabilityReport {
+    pub provider: ProviderKind,
+    pub wire_protocol: ProviderWireProtocol,
+    pub auth_env: &'static str,
+    pub base_url_env: &'static str,
+    pub default_base_url: &'static str,
+    pub tool_calls: ProviderFeatureSupport,
+    pub streaming: ProviderFeatureSupport,
+    pub streaming_usage: ProviderFeatureSupport,
+    pub prompt_cache: ProviderFeatureSupport,
+    pub custom_parameters: ProviderFeatureSupport,
+    pub reasoning_effort: ProviderFeatureSupport,
+    pub reasoning_content_history: ProviderFeatureSupport,
+    pub fixed_sampling_reasoning_models: ProviderFeatureSupport,
+    pub web_search: ProviderFeatureSupport,
+    pub web_fetch: ProviderFeatureSupport,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum ProviderDiagnosticSeverity {
+    Info,
+    Warning,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
+pub struct ProviderDiagnostic {
+    pub code: &'static str,
+    pub severity: ProviderDiagnosticSeverity,
+    pub message: String,
+    pub action: String,
+}
+
 #[allow(clippy::struct_excessive_bools)]
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
 pub struct ProviderDiagnostics {