fix: duplicate request body when when using FULL_DUPLEX_STREAMED ext_proc body mode with retries enabled

[yuehaii]

Commit Message:
when the ExtProc HTTP filter is configured with requestBodyProcessingMode: FULL_DUPLEX_STREAMED and the router has retries enabled, the upstream receives the request body twice. once from the normal ext_proc forwarding path, and once replayed from the router's retry buffer.

Additional Description:
the flag skip_body_on_next_continue_ is added to ActiveStreamDecoderFilter. when set, bufferedData() returns a null pointer which causing doData() to be skipped. the router's retry buffer buffered_request_data_ is never modified, so retry correctness is preserved.

Risk Level:
Low

Testing:

==================== Test output for //test/extensions/filters/http/ext_proc:filter_full_duplex_test:
Note: Google Test filter = HttpFilterTest.FullDuplexStreamedNoBodyDuplicationOnRetryBuffer:HttpFilterTest.FullDuplexStreamedNoBodyDuplicationOnRetryBufferNoTrailers
[==========] Running 2 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 2 tests from HttpFilterTest
[ RUN ] HttpFilterTest.FullDuplexStreamedNoBodyDuplicationOnRetryBuffer
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_quic_always_support_server_preferred_address to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_inject_data_with_state_update to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_return_stop_iteration to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_stream_close_optimization to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_runtime_initialized to: false
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_quic_always_support_server_preferred_address to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_inject_data_with_state_update to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_return_stop_iteration to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_stream_close_optimization to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_runtime_initialized to: false
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_quic_always_support_server_preferred_address to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_inject_data_with_state_update to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_return_stop_iteration to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_stream_close_optimization to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_runtime_initialized to: false
[ OK ] HttpFilterTest.FullDuplexStreamedNoBodyDuplicationOnRetryBuffer (79 ms)
[ RUN ] HttpFilterTest.FullDuplexStreamedNoBodyDuplicationOnRetryBufferNoTrailers
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_quic_always_support_server_preferred_address to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_inject_data_with_state_update to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_return_stop_iteration to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_stream_close_optimization to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_runtime_initialized to: false
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_quic_always_support_server_preferred_address to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_inject_data_with_state_update to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_return_stop_iteration to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_stream_close_optimization to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_runtime_initialized to: false
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_quic_always_support_server_preferred_address to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_inject_data_with_state_update to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_return_stop_iteration to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_ext_proc_stream_close_optimization to: true
[external/abseil-cpp/absl/flags/internal/flag.cc : 148] RAW: Restore saved value of envoy_reloadable_features_runtime_initialized to: false
[ OK ] HttpFilterTest.FullDuplexStreamedNoBodyDuplicationOnRetryBufferNoTrailers (60 ms)
[----------] 2 tests from HttpFilterTest (140 ms total)

[----------] Global test environment tear-down
[==========] 2 tests from 1 test suite ran. (140 ms total)
[ PASSED ] 2 tests.

Release Notes:
fix request body duplicated at upstream when using FULL_DUPLEX_STREAMED ext_proc body mode with retries enabled

Fixes #45470

[gemini-code-assist]

I apologize, but I do not have the capability to trigger a retest of the pull request. As an AI assistant, I can only provide code reviews and summaries. To trigger a retest, please check your CI/CD platform's documentation or use the specific command provided by your repository's CI system.

[yuehaii]

/retest

[tyxia]

@yanjunxiang-google PTAL as well when you have time.