refactor(report-context): extract UPSTREAM_IS_STREAM_KEY constant

The "upstream_is_stream" JSON key flows from the AI execution report
context producer (aether-ai-serving::report_context) through several
consumers — usage runtime metadata copy/move, gateway watchdog, sync
execution decision, observability handlers, and the per-driver usage
repositories. Each site spelled the key as a bare string literal, so a
producer-side rename would silently degrade every consumer to its
fallback (typically assuming streaming) with no compile-time signal.

Introduce a single pub const UPSTREAM_IS_STREAM_KEY in
aether-ai-formats (the lowest crate every consumer already depends on),
re-export from the crate root, and route producer + all map-style
consumers through it. The change is purely a string-literal → constant
swap; behaviour is identical.

Sites left as literals (intentional):
- `json!({"upstream_is_stream": ...})` macro keys, which must be string
  literals at the macro layer; these are also API-response payload
  field names (an external contract that should not silently track
  internal report-context renames).
- SQL column accessors (`try_get::<...>("upstream_is_stream")`), which
  refer to the database schema column, not the JSON key.
- Test fixtures and assertions, which validate the on-the-wire contract
  and should keep verifying the actual string.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/aether-usage-runtime/src/write.rs

@@ -1,5 +1,6 @@
 use std::collections::BTreeMap;
 
+use aether_ai_formats::UPSTREAM_IS_STREAM_KEY;
 use aether_contracts::{ExecutionPlan, ExecutionTelemetry};
 use aether_data_contracts::repository::usage::{UpsertUsageRecord, UsageBodyCaptureState};
 use aether_data_contracts::DataLayerError;
@@ -718,7 +719,7 @@ pub fn build_sync_terminal_usage_payload_seed(
         .report_context
         .as_ref()
         .and_then(Value::as_object)
-        .and_then(|context| context.get("upstream_is_stream"))
+        .and_then(|context| context.get(UPSTREAM_IS_STREAM_KEY))
         .and_then(Value::as_bool)
         .unwrap_or(false);
     let provider_response_full = if upstream_is_stream && payload.body_base64.is_some() {
@@ -1592,9 +1593,9 @@ fn build_runtime_request_metadata_seed_from_parts(
             Value::Bool(client_requested_stream),
         );
     }
-    if let Some(upstream_is_stream) = context_bool(context, "upstream_is_stream") {
+    if let Some(upstream_is_stream) = context_bool(context, UPSTREAM_IS_STREAM_KEY) {
         metadata.insert(
-            "upstream_is_stream".to_string(),
+            UPSTREAM_IS_STREAM_KEY.to_string(),
             Value::Bool(upstream_is_stream),
         );
     }