cpp: Model BDE Base64 and hex pointer-buffer conversions

[kumarak]

Adds taint summaries for both convert overloads of BDE bdlde Base64Encoder, Base64Decoder, HexEncoder, and HexDecoder, propagating input bytes to the output buffer. The models match the member templates by signature and use subtypes: false, so hiding methods in derived classes do not inherit the summary.

API evidence: BDE bdlde headers at ec310b87e008199ecbdbc00a0b0264a53d806a0a under Apache 2.0.

Scope is limited to direct pointer-buffer conversion; stateful flows and endConvert are left for follow-up work.

Validation covers all four classes, overloads, const/mutable inputs, inheritance/hiding behavior, and external-model validation.

[Copilot]

🟢 Approval recommended

The summaries match the referenced API signatures and are comprehensively validated within the stated scope.

Pull request overview

Adds C++ taint summaries for BDE Base64 and hexadecimal pointer-buffer conversions.

Changes:

• Models both convert overloads across four BDE converter classes.
• Adds positive, negative, inheritance, and overload coverage.
• Documents the new analysis support.

File summaries

File	Description
cpp/ql/lib/ext/bdlde.model.yml	Defines conversion summaries.
cpp/ql/lib/change-notes/2026-09-14-bdlde-conversion-models.md	Adds the change note.
cpp/ql/test/library-tests/dataflow/bdlde/bdlde.h	Declares test API fixtures.
cpp/ql/test/library-tests/dataflow/bdlde/test.cpp	Exercises modeled behavior.
cpp/ql/test/library-tests/dataflow/bdlde/flow.ql	Configures taint-flow testing.
cpp/ql/test/library-tests/dataflow/bdlde/flow.expected	Records expected test output.
cpp/ql/test/library-tests/dataflow/external-models/validatemodels.expected	Updates model-validation expectations.

Review details

• Files reviewed: 6/7 changed files
• Comments generated: 0
• Review effort level: Balanced

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