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]>

crates/aether-data/src/repository/usage/memory.rs

@@ -2,6 +2,7 @@ use std::collections::BTreeMap;
 use std::sync::Arc;
 use std::sync::RwLock;
 
+use aether_ai_formats::UPSTREAM_IS_STREAM_KEY;
 use aether_data_contracts::repository::usage::{
     parse_usage_body_ref, usage_body_ref, StoredUsageAuditAggregation, StoredUsageAuditSummary,
     StoredUsageBreakdownSummaryRow, StoredUsageCacheAffinityHitSummary,
@@ -952,7 +953,7 @@ fn usage_output_tps_uses_generation_time(item: &StoredRequestUsageAudit) -> bool
     item.request_metadata
         .as_ref()
         .and_then(Value::as_object)
-        .and_then(|metadata| metadata.get("upstream_is_stream"))
+        .and_then(|metadata| metadata.get(UPSTREAM_IS_STREAM_KEY))
         .and_then(Value::as_bool)
         .unwrap_or(item.is_stream)
 }