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-ai-serving/src/lib.rs

@@ -22,6 +22,7 @@ pub mod request_body_diagnostics;
 pub mod runtime_miss;
 pub mod surface_spec;
 
+pub use aether_ai_formats::UPSTREAM_IS_STREAM_KEY;
 pub use aether_pool_core::{
     normalize_enabled_pool_presets, run_pool_scheduler, PoolCandidateFacts, PoolCandidateInput,
     PoolCandidateOrchestration, PoolMemberSignals, PoolRuntimeState, PoolScheduledCandidate,

crates/aether-ai-serving/src/report_context.rs

@@ -1,6 +1,7 @@
 use std::collections::BTreeMap;
 
 use aether_ai_formats::api::ExecutionRuntimeAuthContext;
+use aether_ai_formats::UPSTREAM_IS_STREAM_KEY;
 use aether_scheduler_core::SchedulerRankingOutcome;
 use serde_json::{Map, Value};
 
@@ -140,7 +141,7 @@ pub fn build_ai_execution_report_context(parts: AiExecutionReportContextParts<'_
         Value::Bool(parts.client_requested_stream),
     );
     object.insert(
-        "upstream_is_stream".to_string(),
+        UPSTREAM_IS_STREAM_KEY.to_string(),
         Value::Bool(parts.upstream_is_stream),
     );
     object.insert("has_envelope".to_string(), Value::Bool(parts.has_envelope));