Block oversized requests before providers hard-fail

The runtime already tracked rough token estimates for compaction, but provider-bound
requests still relied on naive model output limits and could be sent upstream even
when the selected model could not fit the estimated prompt plus requested output.

This adds a small model token/context registry in the API layer, estimates request
size from the serialized prompt payload, and fails locally with a dedicated
context-window error before Anthropic or xAI calls are made. Focused integration
coverage asserts the preflight fires before any HTTP request leaves the process.

Constraint: Keep the first pass minimal and reusable across both Anthropic and OpenAI-compatible providers
Rejected: Auto-compact-and-retry in the same patch | broader control-flow change than the requested minimal preflight
Confidence: medium
Scope-risk: narrow
Reversibility: clean
Directive: Expand the model registry before enabling preflight for additional providers or aliases
Tested: cargo build -p api -p tools -p rusty-claude-cli; cargo test -p api
Not-tested: End-to-end CLI auto-compaction or retry behavior after a local context_window_blocked failure

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

@@ -12,7 +12,7 @@ use crate::types::{
     ToolChoice, ToolDefinition, ToolResultContentBlock, Usage,
 };
 
-use super::{Provider, ProviderFuture};
+use super::{preflight_message_request, Provider, ProviderFuture};
 
 pub const DEFAULT_XAI_BASE_URL: &str = "https://api.x.ai/v1";
 pub const DEFAULT_OPENAI_BASE_URL: &str = "https://api.openai.com/v1";
@@ -128,6 +128,7 @@ impl OpenAiCompatClient {
             stream: false,
             ..request.clone()
         };
+        preflight_message_request(&request)?;
         let response = self.send_with_retry(&request).await?;
         let request_id = request_id_from_headers(response.headers());
         let payload = response.json::<ChatCompletionResponse>().await?;
@@ -142,6 +143,7 @@ impl OpenAiCompatClient {
         &self,
         request: &MessageRequest,
     ) -> Result<MessageStream, ApiError> {
+        preflight_message_request(request)?;
         let response = self
             .send_with_retry(&request.clone().with_streaming())
             .await?;