Commit Message:
When an upstream filter (e.g. envoy.filters.http.buffer) buffers the entire request body before passing it to the router, the shared buffered_request_data_ is passed directly as the data argument to decodeData(). The buffer overflow check computed
decodingBuffer().length() + data.length(), but since both refer to the same object, this double-counted the payload size.

This caused requests with body size between limit/2 and limit to incorrectly trigger buffer overflow, setting request_buffer_overflowed_ and cancelling internal redirects.

The fix detects pointer equality between decodingBuffer() and data, mirroring the existing same-buffer guard in commonHandleBufferData().

tentative fix for #44128
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Request variables

Key	Value
ref	5940bbc
sha	c07cd34
pr	44346
base-sha	94828ca
actor	@wdauchy
message	router: fix buffer overflow double-counting when decoding buffer and data are the same object...
started	1775729960.119927
target-branch	main
trusted	false

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