Add iterator+sentinel tests and docs for binary deserializers (#5265)

* Add iterator+sentinel tests and docs for binary deserializers

This commit extends the C++20 ranges support (iterator+sentinel pairs) to the
binary format deserializers from_cbor, from_msgpack, from_ubjson, from_bjdata,
and from_bson, matching what was already done for parse(), accept(), and
sax_parse().

Changes:
- Add istreambuf_sentinel helper to test_utils.hpp for EOF detection in tests
- Add 5 new test cases that read binary files directly via
  std::istreambuf_iterator<char> + sentinel, without pre-buffering
- Update documentation for all 5 from_* functions to document overload (3)
  with SentinelType parameter
- All tests pass; verified against existing test suite data
- Fix potential buffer over-read warning in heterogeneous iterator test

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Merge iterator+sentinel overloads and fix ambiguity/CI issues

Address PR review feedback and CI failures:

- Merge the separate same-type and sentinel-type iterator overloads of
  parse(), accept(), sax_parse(), and the five from_* binary deserializers
  into a single overload with SentinelType defaulted to IteratorType,
  as suggested in review. Applied the same simplification to the
  detail::input_adapter() free functions.
- Fix a latent ambiguity: some compilers (e.g. GCC 4.8) unreliably SFINAE
  the operator!= detection for std::nullptr_t against container/string
  types, making calls like parse(s, nullptr, ...) ambiguous with the
  compatible-input overload. can_compare_ne now explicitly excludes
  std::nullptr_t as a SentinelType.
- Use a named enable_if_t template parameter instead of an unnamed
  function parameter for the SFINAE guard, fixing a clang-tidy
  hicpp-named-parameter/readability-named-parameter failure.
- Update parse.md, accept.md, sax_parse.md, and the five from_*.md pages
  to document the merged overload instead of separate (2)/(3) overloads,
  also fixing an over-160-char line that broke the documentation
  style_check CI job.
- Rework the BSON iterator+sentinel test to parse a BSON file already
  present in the test suite instead of writing/deleting a temp file.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix -Wunneeded-internal-declaration for CustomSentinel in test

CustomSentinel lives in an anonymous namespace (internal linkage), and
the library's parse loop only ever evaluates the iterator-first
direction (it != last), so the reversed-order friend operator!= was
never referenced. Clang's -Weverything flags such unused internal
declarations as an error. Drop the unused overload; the used direction
is enough to satisfy can_compare_ne's either-order detection.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix clang-tidy hicpp-named-parameter and misc-const-correctness

- Drop the unused reversed-order operator!= overload from
  utils::istreambuf_sentinel (only iterator != sentinel is ever
  evaluated) and name the remaining friend's sentinel parameter, fixing
  hicpp-named-parameter/readability-named-parameter.
- Mark the istreambuf_iterator first/last helper variable const in the
  five binary-format sentinel tests, fixing misc-const-correctness.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix clang-tidy misc-const-correctness in heterogeneous sentinel test

json_str is only read via .data()/.size() and never reassigned, so
clang-tidy correctly flags it as const-able. Verified against the exact
CI job (silkeh/clang:dev, ci_clang_tidy target) by running clang-tidy
directly on this file plus the five binary-format sentinel tests
touched by prior commits; all are now clean.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/from_bson.md

@@ -7,8 +7,8 @@ static basic_json from_bson(InputType&& i,
                             const bool strict = true,
                             const bool allow_exceptions = true);
 // (2)
-template<typename IteratorType>
-static basic_json from_bson(IteratorType first, IteratorType last,
+template<typename IteratorType, typename SentinelType = IteratorType>
+static basic_json from_bson(IteratorType first, SentinelType last,
                             const bool strict = true,
                             const bool allow_exceptions = true);
 ```
@@ -16,7 +16,7 @@ static basic_json from_bson(IteratorType first, IteratorType last,
 Deserializes a given input to a JSON value using the BSON (Binary JSON) serialization format.
 
 1. Reads from a compatible input.
-2. Reads from an iterator range.
+2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).
 
 The exact mapping and its limitations are described on a [dedicated page](../../features/binary_formats/bson.md).
 
@@ -35,6 +35,10 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 `IteratorType`
 :   a compatible iterator type
 
+`SentinelType`
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
+    custom sentinel type for C++20 ranges
+
 ## Parameters
 
 `i` (in)
@@ -44,7 +48,7 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   iterator to the start of the input
 
 `last` (in)
-:   iterator to the end of the input
+:   iterator to the end of the input, or a sentinel value that compares equal to the end iterator with `operator!=`
 
 `strict` (in)
 :   whether to expect the input to be consumed until EOF (`#!cpp true` by default)
@@ -103,6 +107,7 @@ Linear in the size of the input.
 
 - Added in version 3.4.0.
 - Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
 
 !!! warning "Deprecation"