Serialize buffered binary element values to JSON - #2409

Binary element values may be readable, seekable buffers, but Dataset.to_json() passes them directly to base64 encoding and the bulk-data path calls len() on them. Both paths raise TypeError for supported buffered Pixel Data values.

Measure buffered values with buffer_length. If a BulkDataURI handler is selected, pass the original element to it without reading the value. Otherwise, encode the full buffer as InlineBinary and restore its original position. Regressions cover OB/OW inline round trips and a bulk-data buffer whose read method raises, proving that URI serialization does not materialize the data.

Validation: all 3 regressions fail on main before the fix. JSON, data-element, file-writer and file-utility suites pass afterwards (640 passed, 5 skipped, 3 warnings from existing validation tests). Repository-pinned Ruff 0.15.12 lint/format checks and git diff --check pass. Mypy 1.20.2 with Python 3.12 reports six errors in unchanged pixels/utils.py during followed imports; no full type-check pass is claimed. A changed-file check with followed-import diagnostics suppressed is recorded in validation. Python 3.12.14, NumPy 2.5.4, pydicom 3.1.0.dev0. Full package tests and documentation build were not run. A release note is included.