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/anthropic.rs

@@ -14,7 +14,7 @@ use telemetry::{AnalyticsEvent, AnthropicRequestProfile, ClientIdentity, Session
 use crate::error::ApiError;
 use crate::prompt_cache::{PromptCache, PromptCacheRecord, PromptCacheStats};
 
-use super::{Provider, ProviderFuture};
+use super::{preflight_message_request, Provider, ProviderFuture};
 use crate::sse::SseParser;
 use crate::types::{MessageDeltaEvent, MessageRequest, MessageResponse, StreamEvent, Usage};
 
@@ -294,6 +294,8 @@ impl AnthropicClient {
             }
         }
 
+        preflight_message_request(&request)?;
+
         let response = self.send_with_retry(&request).await?;
         let request_id = request_id_from_headers(response.headers());
         let mut response = response
@@ -337,6 +339,7 @@ impl AnthropicClient {
         &self,
         request: &MessageRequest,
     ) -> Result<MessageStream, ApiError> {
+        preflight_message_request(request)?;
         let response = self
             .send_with_retry(&request.clone().with_streaming())
             .await?;