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-formats/src/formats/shared/request.rs

@@ -2,6 +2,14 @@ use base64::Engine as _;
 
 use crate::formats::id::api_format_uses_body_stream_field;
 
+/// JSON key under which `upstream_is_stream` is written into the AI execution
+/// report context and propagated into usage metadata. Shared by the producer
+/// (`aether-ai-serving::report_context`) and every downstream consumer so that
+/// renames cannot silently desync them — a string-literal mismatch here would
+/// degrade to default values (e.g. assuming streaming) without any compile-time
+/// signal.
+pub const UPSTREAM_IS_STREAM_KEY: &str = "upstream_is_stream";
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) enum UpstreamStreamPolicy {
     Auto,

crates/aether-ai-formats/src/formats/shared/response.rs

@@ -3,6 +3,7 @@ use std::collections::BTreeMap;
 use serde_json::Value;
 
 use crate::contracts::core_success_background_report_kind;
+use crate::formats::shared::request::UPSTREAM_IS_STREAM_KEY;
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct LocalSyncReportParts {
@@ -66,7 +67,7 @@ fn should_capture_client_sync_success_body(payload: &LocalSyncReportParts) -> bo
         .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)
 }

crates/aether-ai-formats/src/lib.rs

@@ -29,7 +29,7 @@ pub use formats::shared::model_directives::{
 };
 pub use formats::shared::request::{
     endpoint_config_forces_upstream_stream_policy, enforce_request_body_stream_field,
-    resolve_upstream_is_stream_from_endpoint_config,
+    resolve_upstream_is_stream_from_endpoint_config, UPSTREAM_IS_STREAM_KEY,
 };
 pub use protocol::canonical::{
     canonical_request_unknown_block_count, canonical_response_unknown_block_count,