cpp: model BDE bslx in-stream deserialization taint flow

Add flow summaries for the BDE byte-stream deserializers in
BloombergLP::bslx:

- ByteInStream and GenericInStream<STREAMBUF>: constructing (or, for
  ByteInStream, reset-ing) from a buffer/streambuf taints the stream,
  and every get* method propagates that taint into its output variable.
  get* returns *this, so a fluent row keeps chained calls tainted.
- InStreamFunctions::bdexStreamIn: the generic BDEX entry point that
  populates any bdex-compatible object from a stream, modeled as
  stream -> object.

Not a duplicate of the bsl.* container models or the std/bsl QL models,
which cover only the bsl standard-library-alike types; the bslx BDE
namespace had no coverage. Verified with a BloombergLP::bslx-shaped stub
in the dataflow external-models harness.

Author: kumarak

cpp/ql/lib/change-notes/2026-08-27-bslx-models.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Added flow summaries for the BDE `bslx` byte-stream deserializers (`BloombergLP::bslx`). Constructing a `ByteInStream` or `GenericInStream` from a byte buffer or streambuf taints the stream, each `get*` method (`getInt32`, `getString`, `getArray*`, ...) propagates that taint into the deserialized output (and keeps the returned stream tainted for chained calls), and `InStreamFunctions::bdexStreamIn` propagates taint from the stream to any deserialized object.