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/error.rs

@@ -8,6 +8,13 @@ pub enum ApiError {
         provider: &'static str,
         env_vars: &'static [&'static str],
     },
+    ContextWindowExceeded {
+        model: String,
+        estimated_input_tokens: u32,
+        requested_output_tokens: u32,
+        estimated_total_tokens: u32,
+        context_window_tokens: u32,
+    },
     ExpiredOAuthToken,
     Auth(String),
     InvalidApiKeyEnv(VarError),
@@ -48,6 +55,7 @@ impl ApiError {
             Self::Api { retryable, .. } => *retryable,
             Self::RetriesExhausted { last_error, .. } => last_error.is_retryable(),
             Self::MissingCredentials { .. }
+            | Self::ContextWindowExceeded { .. }
             | Self::ExpiredOAuthToken
             | Self::Auth(_)
             | Self::InvalidApiKeyEnv(_)
@@ -67,6 +75,16 @@ impl Display for ApiError {
                 "missing {provider} credentials; export {} before calling the {provider} API",
                 env_vars.join(" or ")
             ),
+            Self::ContextWindowExceeded {
+                model,
+                estimated_input_tokens,
+                requested_output_tokens,
+                estimated_total_tokens,
+                context_window_tokens,
+            } => write!(
+                f,
+                "context_window_blocked for {model}: estimated input {estimated_input_tokens} + requested output {requested_output_tokens} = {estimated_total_tokens} tokens exceeds the {context_window_tokens}-token context window; compact the session or reduce request size before retrying"
+            ),
             Self::ExpiredOAuthToken => {
                 write!(
                     f,