Cut unused StreamInterceptorContext.h include from ServerGeneratorStreamBridge.h

Summary:
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This diff resolves post-landing build speed regression using recommendations from diff_doctor. Please reach out to oncall+ads_build_speed_official if you have any feedback for this runbook.
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## Summary:
Removed the heavy include `thrift/lib/cpp2/server/StreamInterceptorContext.h` (4.2M bloat) from ServerGeneratorStreamBridge.h and replaced it with a forward declaration. The class is only used through `std::shared_ptr<StreamInterceptorContext>`, which doesn't require the full definition.

This change:
- Reduces include bloat by 4.2M (15.0M -> 19.2M)
- Reduces includes by 216 (1066 -> 1282)
- Improves build speed by reducing compilation dependencies
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Reviewed By: vitaut

Differential Revision: D92555145

fbshipit-source-id: b491237ef43290a1734b01e6e2e222bea7b4aff6

Author: generatedunixname893464919433493

third-party/thrift/src/thrift/lib/cpp2/async/ServerGeneratorStreamBridge.h

@@ -30,9 +30,10 @@
 #include <thrift/lib/cpp2/async/StreamMessage.h>
 #include <thrift/lib/cpp2/async/TwoWayBridge.h>
 #include <thrift/lib/cpp2/async/TwoWayBridgeUtil.h>
-#include <thrift/lib/cpp2/server/StreamInterceptorContext.h>
 
 namespace apache::thrift::detail {
+class StreamInterceptorContext;
+
 namespace test {
 class TestProducerCallback;
 }