fix(api): restore local preflight guard ahead of count_tokens round-trip

CI has been red since be561bf ('Use Anthropic count tokens for preflight')
because that commit replaced the free-function preflight_message_request
(byte-estimate guard) with an instance method that silently returns Ok on
any count_tokens failure:

    let counted_input_tokens = match self.count_tokens(request).await {
        Ok(count) => count,
        Err(_) => return Ok(()),  // <-- silent bypass
    };

Two consequences:

1. client_integration::send_message_blocks_oversized_requests_before_the_http_call
   has been FAILING on every CI run since be561bf. The mock server in that
   test only has one HTTP response queued (a bare '{}' to satisfy the main
   request), so the count_tokens POST parses into an empty body that fails
   to deserialize into CountTokensResponse -> Err -> silent bypass -> the
   oversized 600k-char request proceeds to the mock instead of being
   rejected with ContextWindowExceeded as the test expects.

2. In production, any third-party Anthropic-compatible gateway that doesn't
   implement /v1/messages/count_tokens (OpenRouter, Cloudflare AI Gateway,
   etc.) would silently disable the preflight guard entirely, letting
   oversized requests hit the upstream only to fail there with a provider-
   side context-window error. This is exactly the 'opaque failure surface'
   ROADMAP #22 asked us to avoid.

Fix: call the free-function super::preflight_message_request(request)? as
the first step in the instance method, before any network round-trip. This
guarantees the byte-estimate guard always fires, whether or not the remote
count_tokens endpoint is reachable. The count_tokens refinement still runs
afterward when available for more precise token counting, but it is now
strictly additive — it can only catch more cases, never silently skip the
guard.

Test results:
- cargo test -p api --lib: 89 passed, 0 failed
- cargo test --release -p api (all test binaries): 118 passed, 0 failed
- cargo test --release -p api --test client_integration \
    send_message_blocks_oversized_requests_before_the_http_call: passes
- cargo fmt --check: clean

This unblocks the Rust CI workflow which has been red on every push since
be561bf landed.

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

@@ -487,10 +487,21 @@ impl AnthropicClient {
     }
 
     async fn preflight_message_request(&self, request: &MessageRequest) -> Result<(), ApiError> {
+        // Always run the local byte-estimate guard first. This catches
+        // oversized requests even if the remote count_tokens endpoint is
+        // unreachable, misconfigured, or unimplemented (e.g., third-party
+        // Anthropic-compatible gateways). If byte estimation already flags
+        // the request as oversized, reject immediately without a network
+        // round trip.
+        super::preflight_message_request(request)?;
+
         let Some(limit) = model_token_limit(&request.model) else {
             return Ok(());
         };
 
+        // Best-effort refinement using the Anthropic count_tokens endpoint.
+        // On any failure (network, parse, auth), fall back to the local
+        // byte-estimate result which already passed above.
         let counted_input_tokens = match self.count_tokens(request).await {
             Ok(count) => count,
             Err(_) => return Ok(()),