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 <[email protected]>

apps/aether-gateway/src/execution_runtime/sync/execution.rs

@@ -3,6 +3,7 @@ use std::io::Error as IoError;
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
+use aether_ai_serving::UPSTREAM_IS_STREAM_KEY;
 use aether_contracts::{
     ExecutionError, ExecutionErrorKind, ExecutionPhase, ExecutionPlan, ExecutionResult,
     ExecutionTelemetry,
@@ -1005,7 +1006,7 @@ fn resolve_openai_image_sync_total_timeout_ms(plan: &ExecutionPlan) -> u64 {
 
 fn report_context_upstream_is_stream(report_context: Option<&Value>) -> bool {
     report_context
-        .and_then(|value| value.get("upstream_is_stream"))
+        .and_then(|value| value.get(UPSTREAM_IS_STREAM_KEY))
         .and_then(Value::as_bool)
         .unwrap_or(false)
 }