feat(coding-agent): add before_provider_headers extension hook (#6350)

* feat(coding-agent): add before_provider_headers extension hook

Extensions can already rewrite the request payload through before_provider_request, but there is no way to adjust the outgoing HTTP
headers of a provider call. This hook fills that gap for cases like request tracing, session correlation, or tenant routing.

Handlers mutate the headers map in place - a null value deletes a header - and the return value is ignored, so a handler cannot accidentally drop auth or attribution headers by
forgetting to spread.

* docs(coding-agent): document before_provider_headers extension hook

Add the before_provider_headers section and lifecycle-diagram entry to
extensions.md; drop the now-superseded proposal file.

packages/coding-agent/src/core/sdk.ts

@@ -313,6 +313,19 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			const timeoutMs = options?.timeoutMs ?? providerRetrySettings.timeoutMs ?? effectiveTimeoutMs;
 			const websocketConnectTimeoutMs =
 				options?.websocketConnectTimeoutMs ?? settingsManager.getWebSocketConnectTimeoutMs();
+			let headers = mergeProviderAttributionHeaders(
+				model,
+				settingsManager,
+				options?.sessionId,
+				auth.headers,
+				options?.headers,
+			);
+			// Let extensions inject/adjust per-request headers (e.g. tracing, session correlation)
+			// after static assembly, before the provider HTTP call.
+			const headerRunner = extensionRunnerRef.current;
+			if (headerRunner?.hasHandlers("before_provider_headers")) {
+				headers = await headerRunner.emitBeforeProviderHeaders(headers ?? {});
+			}
 			return streamSimple(model, context, {
 				...options,
 				apiKey: auth.apiKey,
@@ -321,13 +334,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 				websocketConnectTimeoutMs,
 				maxRetries: options?.maxRetries ?? providerRetrySettings.maxRetries,
 				maxRetryDelayMs: options?.maxRetryDelayMs ?? providerRetrySettings.maxRetryDelayMs,
-				headers: mergeProviderAttributionHeaders(
-					model,
-					settingsManager,
-					options?.sessionId,
-					auth.headers,
-					options?.headers,
-				),
+				headers,
 			});
 		},
 		onPayload: async (payload, _model) => {